George Bailey dies before the end of the film.See this post for details.
Mr. Potter is a muggle relative of Harry Potter. All the money that Harry has came from Bedford Falls!
Clarence takes George to an alternate universe where he was never born.Clarence is an angel. He hasn't gotten his wings yet (which goes with added abilities), but perhaps he can take people into alternate universes if the situation calls for it. It's not that he changes history so that George wasn't born. There's an alternate universe in which George was never born, and Clarence took George to that universe to see what would have happened had he never been born.
When Potter heard about George being bailed out by his friends and family, he gets so angry that he has a heart attack and dies in agony.
The "lost ending" for this film aired on Saturday Night Live and replayed on all the SNL Christmas specials is/was a genuine lost ending.Not quite as nice a thought, but it does fix the "Potter is a Karma Houdini" problem. The classic ending is suspected of Getting Crap Past the Radar; the extra footage may have been filmed just in case.
The "angels" are actually god-like beings that humans cannot perceive.For whatever reason, George falls into their plans, and if he dies, he will not be able to fulfill them.
When Mary broke the glass after the dance, she wished for George to have a miserable life.He received news that his father died immediately afterwards—things went downhill from there.
Most of what happens in the world where George isn't born is fabricated by the angels.Rather than played out to their most likely hypothetical potential, several occurrences in the alternate universe are exaggerated or completely falsified to persuade George of his significance.
The fact that Harry's gravestone in the alternate reality reads 1911-1919 is not a mistake.At least not a mistake on the on the filmmaker's part. Pottersville is so crappy that when Harry died and his parents ordered the gravestone, those responsible for chiseling in the dates put in the wrong year for either Harry's birth or Harry's death. His parents were upset but the stone was so expensive and Pottersville so crappy they couldn't afford to replace it.
Mr. Potter is really SatanThink about it, Pottersville is basically the 1940s equivalent to hell. He tempts people with what seems to be easy deals for them but with an ulterior motive. Bailey even implied that if they sold out to Potter they may as well be selling their souls.
Prosperity is right around the corner for the BaileysWorld War II is coming to an end. The United States is coming out of its economic depression. The prosperous 1950's are about to begin, and people are going to start buying land and houses. George is already an established figure in the local real estate industry, and his business has a good reputation as a lender. He's in the perfect position to take advantage of a market boom. Soon he'll own Bedford Falls.
Three years ago Violet Bick and Ernie the Cab Driver had an affairGeorge was both able to talk Mrs. Bishop out of divorcing Ernie and Ernie into ending things with Violet. Violet, despondent and now a pariah for being a (potential) home wrecker, finally decides it's better leave Bedford Falls, hence her asking George for money on Christmas Eve so she can start over in New York. This is also why in the Pottersville timeline Ernie's wife "ran away three years ago and took the kid"; George wasn't there to intervene and save the marriage.
Mr. Gower's son is now a member of the Cullen clan.The telegram indicates that he was living away from Bedford Falls at the time of his death, and he died from the same illness that Edward Cullen was dying of. He also looked to be around Edward's age, if Mr. Gower's photograph was recent. Who's to say he couldn't have been in Chicago, in the same hospital? (At least it's what I tell myself to make H.B. Warner's performance less heartwrenching.)
Mr. Potter's middle inital is FI wonder what that stands for...
Mary's plainer appearance in Pottersville is a personal trick.Mary still is the attractive woman George knew. Sure, the stress of living in Pottersville took a little of it out of her, but not much. Most of her frumpiness is put-on, in order to avoid any unwanted attention from men around the town.
Potter made the same wish that George did.Related to a theory listed above, Potter got angry that George was so happy and successful. He wished that George was never born. Suddenly, a fallen angel appears and agress to tell Potter how to make the wish come true, if he'll let his soul get taken to Hell upon his death. Potter agrees, and the angel tells Potter about Uncle Billy and the money, and how to really tighten the screws on George and utterly destroy him, resulting in George's wish and how Clarence will unknowingly grant it and give Potter what he wants. It works, and the angel tells Potter about it. But it turns out the forces of Heaven give George a chance to take back the wish, which he does. When Potter sees his wish has been foiled, he dies of that heart attack. And, since the deal just involved advising Potter, not necessarily guaranteeing success, the fallen angel shows up at Potter's death and takes him to his richly deserved damnation.