Ascended Fanon: Due to how little the final film bore to the source material, many fans of Frozen had theorized that Andersen's Snow Queen existed in the Frozen-verse as a separate entity than Elsa, another fan theory being that Elsa's powers were tied back to the former. Once appears to be going with an idea similar to this, with Elizabeth Mitchell portraying the Snow Queen as a separate character from Georgina Haig's Elsa.
The Danza: Young princess Eva is played by Eva Allan (also credited as Eva Bourne).
Defictionalization: The exterior shots of Storybrooke's main street are filmed on a real road in British Columbia. The signs on certain shops are covered with a Storybrooke equivalent during filming and removed afterwards. However, some shop owners let the production company keep the signs up permanently partially due to this trope and partially to make less work for the film crew (to erect and tear down the signs every week). The producers also leased two units just for the storefronts. These are kept the same year round, since they were never real shops in the first place.
Dueling Shows: With Grimm, an NBC show that also focuses on fairytale characters existing in the real world, although that show focuses on more obscure characters and creatures from the many, many lesser known Grimm fairytales, as opposed to the more famous and iconic ones in Once.
The Queen's fans are called Evil Regals, Snow White's are the Fairest, the Seven Dwarves' are Team 7, Prince Charming's are Charmers, Ruby's are Little Reds, and Belle's are Beauties. Emma's fans were originally called "Team Emma" though "Ugly Ducklings" is becoming more popular.
From LOST: Charles Widmore is Prince Charming's father; Claire Littleton is Belle; Hurley is Anton; Charlotte is the Wicked Witch of the West (Ben was right to try to shoot her...); and as of "White Out" Juliet looks to be the season 4 Big Bad.
Erica Sayers is Regina's controlling ambitious mother. So... Regina and Nina are siblings?
I Knew It: Mr. Gold perfectly remembers being Rumplestiltskin. Most fans probably guessed it from a mile away, though The Reveal was no less satisfying.
Surprise (or not)! Cora is the Queen of Hearts after all.
Several people guessed as early as the first season that Henry's father (Neal) is Baelfire.
Fan boards had joked since Frozen hit theaters about how it would translate to that universe. Guess who just showed up...
Many had theorized that Elsa wasn't the actual Big Bad of Season 4 and that the Snow Queen was the real villain
Older Than They Think: Little Red Riding Hood hunting a werewolf that terrorises her village every month at the full moon? That's the plot of a film called Red Riding Hood with Amanda Seyfried as the heroine. In both movies the love interest is a suspect and both movies call him Peter to boot (possibly as a Shout-Out to Peter and the Wolf). In both stories Red's father was a wolf. Except in the film he's the main antagonist while in the show he's passed it on to Red herself.
The Other Darrin: It seems to be intentional. In Season 1, the Queen of Hearts is played by Jennifer Koenig. In Season 2, she's played by Barbara Hershey when she's revealed to be Cora. It should be noted that even when she was played by Jennifer Koenig, Barbara Hershey supplied the voice, which was hard to make out due to her speaking only in whispers.
In season 3, Sean Maguire replaces Tom Ellis in the role of Robin Hood.
Jennifer Morrison and Sebastian Stan, which is more creepy than the above example when their characters' interactions are considered. They broke up in 2013.
Shipper on Deck: Ginnifer Goodwin (Snow White), Emilie de Ravin (Belle), and Michael Raymond-James (Neal/ Baelfire all ship Swan Thief (Emma/Neal).
Jennifer Morrison (Emma) and Colin O'Donoghue (Hook) both ship Captain Swan (Emma/Hook). Josh Dallas (Prince Charming) seems to be on board as well.
Also, considering his overjoyed expression at their wedding, most fans have assumed that Archie/Jiminy ships Rumbelle...
Tinkerbell is a not so subtle shipper of Outlaw Queen (Regina/Robin)
Throw It In: During the Ruby/Granny fight in "Red Handed" August can be seen all, "These ladies crack me up." His actor was clearly trying not to laugh, but it was so funny the scene was added.
Unreliable Expositor: Emma claims to have been 18 when she had Henry, but he was already 10 when she turned 28, meaning she had to have been 17 when she gave birth to him.
In Manhattan (which at the most is 6 months after the pilot) Henry is 11, making Emma 16 when she was pregnant.
Vindicated by Reruns: A common occurrence through DVR ratings. Example: the first half of Season 3's Neverland arc (episodes 1-6) was a ratings success on all fronts, but the second half not so much. A rerun of its final episode, however, brought its ratings score up to standard.
Apparently they wanted Lady Gaga as the Blue Fairy.
The Sheriff was meant to be Sherlock Holmes, and his curse in Storybrooke was that there was no mystery to solve. The plan fell through due to an issue with acquiring the rights. Not to mention the fact that there are already twoSherlocks on TV right now.
Ariel and Aladdin were planned for the second season, but their appearances got "pushed back" for unknown reasons. (Ariel showed up in Season 3, but Aladdin has yet to appear).
Early drafts of the pilot episode had Charming killed off. This was changed after Josh Dallas auditioned and "blew them away" with his performance, prompting the creators to expand his role.
Early drafts also had Regina with three children instead of just Henry, but the identities of the other two are not yet known.
Hook was supposed to have a larger role in the second half of season two, but it had to undergo a major rewrite due to Colin O'Donoghue breaking his leg. The writers had to cut almost all of Hook's scenes.