Creative Differences: Penders vs. Bollers got so bad, it ended with them both leaving the comic simultaneously. According to Penders, the editor of the comic forced him to bail because he kept on changing his stories, the final straw being the Mobius: 25 Years Later story. More specifically, he stated that there would be no more work done on said story until Penders included other characters from the games, such as Shadow. Penders quit in response.
Creator Backlash: Per word of Aaron Webber, Archie in general seems to pretend that their Sonic comics never existed once they lost the license, due in part to them and Sega apparently departing on sour terms and Archie wanting to move on from the series anyway.
Creator's Favorite: Several writers have admitted that Princess Sally Acorn is their favorite character.
Executive Meddling: The series had a lot of issues of Archie suffering this under Sega. To date, here is a known list of all the official mandates Sega has imposed on the comics, along with other baggage they've brought into the comics.
Certain characters, including certain ones from the games, are off limits for use, such as Eggman Nega (who is not only barred from unobstructed appearances, but can't even be fully named on-panel) Mephiles the Dark and Black Doom (although the latter is still allowed to cameo in flashbacks and the Black Arms race is free for use). It also took years for characters like Cream the Rabbit and her mother to be allowed to appear in the comic, same with Emerl and Omega. Certain characters like Blaze the Cat and Marine the Raccoon had so many restrictions with what could or couldn't be done with them that the most they could do was just make cameo appearances.
Game characters can not have parents or relatives. This also caused the Continuity Reboot to change things up, such as the fact that Uncle Chuck isn't blood related to Sonic anymore, and that the title is honorary.
Sonic cannot get overly emotional (which is often paraphrased or misinterpreted as "Sonic can't cry.").
The name "Mobius" cannot be spoken or mentioned as the name of the planet, and it is not allowed to be called "Earth" either—Sega says it is now just "Sonic's World".note They still call themselves "Mobians" though.
Originally, the SEGA cast couldn't be implied to have aged to the point where they are now or age further. The Classic forms of characters couldn't be implied to be younger forms of the characters either. This mandate has been officially lifted as of the comics reboot, as the Sonic Mega Drive comic uses the Classic designs as the characters younger counterparts (and not long before that, a post-reboot issue featured a flashback to the proto-Silver Sonic that used Sonic's classic design).
Characters from Sonic Underground, the OVA, and Sonic X cannot make an appearance. According to head writer Ian Flynn, this is due to Sega not being keen on giving attention to any past incarnations of the franchise. As such, they forbid the use of any character from the aforementioned pieces of media.
Part of why Knuckles and the echidna race had so much build-up back in the day is that Sega vetoed ideas that were originally meant for Sonic, yet went through with no problem when applied to Knuckles.
As of the reboot, every non-Sega, SatAM, AoStH, and Ian Flynn character has been removed until further notice. While he hints that he can use Sonic Underground, he didn't use characters like Manic and Sonia without their in-series background of being Sonic's siblings in the end, mostly because he knows the Underground fandom would get up in arms.
Ian originally wanted the arc leading up to Issue 250 to be about the Freedom Fighters rescuing Sally with her being de-roboticized in the milestone issue. However, due to the Worlds Collide crossover, these plans were aborted and Sally was returned to normal thanks to the reboot.
All of the 170+ characters created by writer Ken Penders have been permanently removed from the series due to a major lawsuit with him, including several major characters and series mainstays.
While Eggman Nega technically does exist in the Archie series, he is not allowed to make an unobstructed appearance or even be fully named (only being referred to as "Doctor Nega") because of unexplained hangups with Sega. Also affected by unusual hangups are Tiara Boobowski and her father Gazebo, from the canceled game Sonic Xtreme.
Black Doom and Mephiles are off-limits for similarly unexplained reasons (though it could be because they are both dead in canon). The former caused Ian Flynn to create Black Death.
Tundra's true identity was guessed by quite a few people.
Once the the comics hit a delay at the start of 2017, many called that the comics were likely cancelled. On July 2017, that was indeed confirmed.
When Sega announced that the comic was seeking a new publisher, many guessed that IDW Publishing was be a likely candidate due to having hosted many a property (Hasbro, many Cartoon Network series, etc). Sure enough, Sega announced that they had signed a deal with them. 2 days after their announcement of the canceled Archie comic.
The Sonic/Image crossover is at best unlikely to be reprinted, and so is the first story in Sonic Live.
With Archie losing the Sonic license outright, it's safe to assume that the entire series will be in this, at least for now.
Name's the Same: Black Death, the Black Arms alien from the new continuity, shares the same name as a one-off Mobian villain from the previous continuity.
No Export for You: Due to the legal mumbo-jumbo the whole comic has suffered along the years, not to mention the lack of popularity of the video games outside animated adaptations in that region, the comics aren't translated in Latin America.
For a long time, it was impossible to find the series in the United Kingdom as well, because the license didn't cover the territory. Instead, Fleetway used to produce Sonic the Comic.
Old Shame: Though not the writer on the story, Jonathan "WB" Gray, one of many a Promoted Fanboy, absolutely regrets being the guy who drew "The Slap." He has expressed similar resentment towards the "Sonic's Angels" story, a story which infamously depicts what can only be described as borderline Torture Porn being performed on poor Bunnie.
Print Long-Runners: The longest-running comic book based on a licensed property, video game or otherwise, ran from November 1992 to July 2017; 24 years.
Promoted Fanboy: Several examples, most prominently current head writer Ian Flynn who was known as the author of a popular fan comic Other-M before getting hired as head writer.
Recursive Adaptation: The comic had already started to incorporate some plot elements from Sonic Chronicles, which itself had seemingly borrowed and modified some of its echidna lore material from the comic.
The trade paperbacks. Most recent ones have been pushed back at least once, and some have been outright cancelled. The last one came out in April. Read more in this (very long) thread.
Between late 2015 and early 2016, issues of the main series and Sonic Universe were taken off of their original release dates and postponed for undisclosed reasons.
Early 2017 likewise saw a slip in the schedule of both series as well as the "Mega Drive" mini series.
With Archie losing the comic license and being given to IDW, it's unlikely any stories that were caught in this will ever get out of it.
Screwed by the Lawyers: The ongoing lawsuit by Ken Penders led to the vast majority of the cast up and disappearing. As of #252, the entire universe rebooted, with 256 establishing that the whole multiverse implodes on itself and was rebuilt. The new universe gradually but completely overwrote the old, including memories, so the old world was completely gone.
These legal problems came to a head in July 2017, when SEGA just up and cancelled the comic outright.
Shrug of God: When asked about the fate of Robotnik's parents, Flynn posted two ideas. One was that Robotnik "did away" with them. The other was that they migrated to the United Federation. In either case, the writer leaves it up to the reader.
What Could Have Been: The original plot for "Endgame" really did involve Sally's demise — her apparent restoration later on was to have been a robot double, the truth only slowly coming out later as "Sally" began to act increasingly suspicious and out-of-character.
Ken's stated that some of the original ideas for the Princess Sally miniseries included a roboticized Queen Alicia ruling over her own subjects as a threat to both the Freedom Fighters and Robotnik. Another was a Thelma and Louise type story with Sally and Bunnie. Sega vetoed both of these ideas, wanting to use only Sally and Robotnik. This is what led to the Substitute Freedom Fighters and Geoffrey St. John. Regarding the Source of All, one of the ideas that Ken was brainstorming with a friend of his was that the Source was actually an AI made out of a pool of nanotechnology, from either the past or the future, that was so advanced as to be indistinguishable from magic. However, they never figured out what its motives were before it was nuked.
Both Karl Bollers and Ken Penders have made public their plans for the comic had their employment at Archie continued.
When Ian Flynn took over as main writer, he decided to quickly wrap up a couple of leftover plots point, leading to some cases of Fauxshadowing.
Robotnik at one point had problems with an unseen person who called themselves "Anonymous", showed themselves very adapt at reprogramming robots and computers, and Robotnik noticed that "Anonymous" from all evidence appeared to about as tall as himself. The idea was that this "Anonymous" would later be revealed to be Robotnik Prime, who was Back from the Dead for real this time. Instead, it was revealed it was actually A.D.A.M. who was behind it all.
The end of "The Good, the Bad, and the Unknown" arc was very heavily setting up that Ivan Kintobo, an ancestor of Robotnik and creator of Shadow, had survived as a Human Popsicle and implied that he was going to be a major antagonist. The comic would later reveal that Ivan was actually long dead as he had not survived being frozen down.
Ian Flynn originally intended to have E-123 Omega off Tommy Turtle, but Executive Meddling had that changed to the Egg Fleet. In fact, Executive Meddling kept Omega from appearing in the comic for some time since Karl Bollers tried to put him in. It wasn't until issue 3 of Sonic Universe that he made an appearance.
One of the biggest cases of this was the Endangered Species arc. Part one had font that was obviously put in at the last minute to keep from referring to certain characters by name, while the rest was completely rewritten to remove them altogether. The arc was controversial at best as a result.
Flynn has also gone on record to say that the early implied romance between Tails and Barby Koala (of the Downunda Freedom Fighters) is "completely one-sided and kind of creepy", (most likely due to the age difference, with Tails being around 11-12 and Barby appearing to be in her late teens) and that as long as he's on the comic it's not going to happen.
Currently, Ian Flynn is writing an essay known as the "Lost Hedgehog Tales," which will explain his original plan for the series, before the Ken Penders lawsuit, leading up to issue 300.
Chapter 1: Antoine was originally going to eventually die to Metal Sonic's explosion, but was saved by the fan reaction to his injuries. Also, Hershey the Cat wasn't dead after all.
Ian Flynn really wanted to write a conclusion to Sonic Underground, but Executive Meddling always got in his way. He came close to being able to release the story in the fiftieth issue of Sonic Universe, with official promo art even being released, but those plans were nixed in favor of writing a lead up story to Worlds Collide. And thanks to the Cosmic Retcon destroying the series' multiverse, the chances of the story ever being released are unlikely. However, Ian Flynn intends to include the story among other scrapped story plans in the Lost Hedgehog Tales.
Ken Penders got around the CCA somehow by hinting at one of the characters in the "Mobius: X Years Later" storyline being gay... and somehow doing this while showing the life partner, threading the relationship into the text. Once fans started searching, it became obvious (and confirmed by Ken), that the hinted couple was Rotor and Cobar, the latter an echidna scientist from the Dark Legion.
In the new continuity, Gold the Tenrec was confirmed to be either bisexual or lesbian (her creator hadn't decided yet).
Current writer Ian Flynn has organized his own message board, where he and other creators interact with fans and answer their questions. His predecessor, Ken Penders, also kept up a message board while he was writing for the comic, which is still active and occasionally talks about the stories that he had planned for the series.
The events of Sonic the Hedgehog (2006) occurred in the comic as they did in the game, including the Cosmic Retcon. Soleanna has been mentioned a few times and issue #246 has a brief appearance of White Acropolis. The Complete Sonic Comic Encyclopedia describes it in more detail, including entries on Iblis, Mephiles, and Solaris.