WARNING: This page contains no small number of spoilers, but many of the examples below will assume you have knowledge of the spoilers revealed by the endings of the first four films — and as such, those spoilers frequently go untagged. You Have Been Warned!
Ascended Fanon: For years, it had been a theory that the man limping and putting the key behind Michael's eye in the video shown in Saw II was Lawrence Gordon. Fans also seemed convinced that he assisted in other traps that required medical knowledge. In Saw 3D, this became canonical (despite director Kevin Greutert's original desire to have him get revenge against Jigsaw and his accomplices).
Billing Displacement: Tobin Bell is always top-billed in the later sequels, despite being dead in the "present" and only appearing in a handful of flashbacks and tapes. The displacement is taken to ridiculous lengths in the seventh film, where Bell is top-billed in spite of being in the film for TWO MINUTES; Cary Elwes appears in three scenes, has four lines of dialogue, and yet he is billed above Sean Patrick Flannery, who, at fifth billing, has the most screentime of anyone in the film and is essentially the main "hero" character; Chad Donella, who appears throughout the film and is the lead cop in the movie, doesn't even make the top five or the posters; Costas Mandylor and Betsy Russell get second and third billing, respectively, even though the former is barely seen until the end, and the latter, while having more screentime that Mandylor, doesn't do a heck of a lot - their billing can be pinned on their being staples of the series, having appeared in every film since Saw III, rather than their roles in the plot.
Enforced Method Acting: For the Water Cube Trap in Saw V, Scott Peterson really was locked in a cube that slowly filled with water. He had hand signs so that when he lost the ability to continue holding his breath, crew members could quickly run in and drain the water.
Saw III was intended by the writers to be the swan song for the series. The producers wanted more sequels, and had the writers thus make changes to the film to leave room open for future installments. Many of these were fairly minor, such as a shot of Jigsaw pouring wax over a tape that later played into Saw IV. The biggest change, however, was that of Eric Matthews' death. In the original cut, Amanda was supposed to have stabbed him in the neck with a knife, solidifying her transition to a full-out murderer killing out of malice (she had previously killed Adam as a way of mercy-killing). The producers forced director Darren Lynn Bousman to cut it out to leave room for Matthews' return in a later sequel. Thus, the scene now ends abruptly with Amanda simply walking over towards Matthews, and an ADR line of Jigsaw later saying to Amanda that she had merely left him for dead was added. Keep in mind that Donnie Wahlberg (who played Eric) only showed up for Saw III with the promise that he would be killed off, so he was pretty pissed off when they cut out his scene. Darren had to beg him to come back for Saw IV.
A massively cunning, yet very dickish move by Lionsgate and Twisted Pictures, happened two weeks before the shooting of Saw VII began - they contractually forced the director of Saw VI, Kevin Greutert, to return and direct Saw VII while having David Hackl, the original director, shifted over to a yet-unconfirmed Lionsgate movie. This was because Greutert was originally set to direct the sequel to Paranormal Activity, which was scheduled to be released on the same date as Saw VII in a similar situation to how the original film contended with Saw VI in 2009. Needless to say, both Greutert and Hackl didn't take that move very well.
Costas Mandylor, who plays Hoffman in Saw III to Saw 3D, is also Australian and impressively, his accent never slips noticeably once throughout the five movies.
Angus Macfayden (Jeff Denlon) is Scottish, and he does an admirable job of keeping his natural accent from slipping too much.
Matt Passmore, who plays Logan in Jigsaw, is yet another Australian, who also pulls off a reasonable accent.
Harpo Does Something Funny: According to DVD Commentary, Tobin Bell wrote large parts of his character's dialogue while they filmed the movies, to the point that scripts from some of the movies would simply feature "Insert Tobin Bell dialogue" in several scenes that featured Tobin.
I Knew It!: In Saw 3D, it's revealed that Dr. Gordon survived the first film and had been disciple of Jigsaw working behind the scenes since then. Some fans theorized this originally based on a brief part of the video shown to the key-behind-the-eye victim in Saw II. A cloaked figure with a limp (assumed later to be Jigsaw) is seen next to an operating table. Saw 3D reveals that this WAS in fact Dr. Gordon.
Irony as She Is Cast: Despite being best known for his part in the franchise, Tobin Bell says that he isn't and never has been a fan of horror films.
Meme Acknowledgement: The official Twitter account for the movies have acknowledged the existence of the shitpost account "@jigsaw_quotes".
No Stunt Double: As seen in behind the scenes segments on how the traps are made, plenty of the traps (even the most dangerous) had the actor themselves or a stunt double in the trap with special precautions in place. Namely, in Saw V, it was really Joris Jarsky (Seth Baxter) and Scott Patterson (Peter Strahm) in the Pendulum Trap and Water Cube.note Which had the former's lower body sitting in a strategically placed hole while a real pendulum cuts his pretend foam/rubber body and the latter's head was placed inside of a glass cube filled with water with two accessible doors until he signaled to be let out. It was said that the latter trap had to have him instead of a stunt double out of realism for the scene.
Word of God: According to the writers' commentary track on the Saw 3D DVD, the two men with Dr. Gordon when he captures Hoffman at the end of the film are Brad and Ryan, the two surviving parties of the film's opening trap.
Dummied Out: In the final two chapters of the game, you see some sewing kits. It looks like they would be tools to help make traps, but none of the ones you can make require them. It's likely there were more traps that didn't make it to the full game. Notably, you can't actually pick up the sewing kits except for one near the end which does nothing.