There is a post by Revy somewhere that adked the same question. Hillarious... most beers people think they have ruined turn out to be their best beers.
Seriously, though, once you are on the forums long enough, you notice that about 80% of those who think they ruined their beer end up with perfectly good beer. About 19.2% make a huge user error that causes their infection or other problem, even if they have no idea what that error was. The other 0.8% actually have screwed up beer through no fault of their own.
It is difficult to mess up a batch of beer if you practise good santitation and proven recipes!