I was thinking about how somewhat simple straightforward origins morph into these complex things.

Batman's parents were originally just two people in the wrong place at the wrong time. Then we got a name added to the mugger, Joe Chill. Then a revelation that Chill had been hired to kill Thomas Wayne, due to Wayne's testifying against crime boss Lew Moxton. Then it was revealed that Chill's mother had been the housekeeper for Bryce's uncle and had helped raise Bruce after the murder. Recently there was the whole Court of Owls stuff.

Luthor was a weapons dealer with a full head of red hair who was manipulating the two sides in a war for profit and with an eye to gaining political power. He had no particular beef with Superman, it was just that Superman was the hero who stumbled on the scheme and stopped it. Later his backstory was expanded to knowing Superman since childhood and he was retroactively bald from this time. He gained the first name Lex. He was given a sister (though she never knew she was related to him). Post-Crisis we were told Lex had killed his own parents for insurance money which he used to start his company. This later was changed so Lex inherited money from his equally amoral father, Lionel based on the Smallville TV show.

Barry Allen went from a random police scientist who had a lab accident and gained super-speed to the embodiment of the Speed Force that powers every speedster. His rather average life prior to gaining his powers is now a tragic backstory caused by an enemy he hadn't even met yet. At one point we were told he had a twin brother whose family line would lead to Barry's greatest enemy, the time travelling Reverse-Flash.

Those are just a few examples.

So what changes to a character's origin/back story do you like … and which ones are just a bridge too far?