There are plenty of mistakes in the series, and Rowling admits that they’re there. Most of them involve math, which she claims to be very bad at, although there are a couple of non-math errors (the most glaring being in the first-printed version of the Goblet of Fire, in the graveyard scene – Harry’s parents come out of the wand in the wrong order – she corrected that in later versions).
I noticed another one yesterday, re-reading Philosopher’s Stone: Harry knows where the kitchens are, while he doesn’t in the 4th book, making me wonder if she had the Hogwarts House Elves planned at all in the early versions.
If you want to talk major plot holes, it never really is explained why, in Goblet of Fire, pseudo-Moody doesn’t just turn a chocolate bar into a portkey to get rid of Harry. There are lots of explanations – the best being that probably that he wanted to make it look like an accident so that Voldemort could return quietly – but none has ever been given by Rowling herself.
As for time-travel, Rowling’s approach is that all changes that will be made have been made, so, obviously, no one will save Harry’s parents. Some of the most bizarre theories I’ve read involve Harry being at Godric’s Hollow the night his parents died – that is, time-turnered Harry, as an adult. The most warped theories involve him kiling his parents as part of some accident or to fulfil some cosmic necessity. :eek:
Rowling can be pretty sadistic to her main character, but I don’t think even she’d go that far.