When you've been without a clue for ages, life just isn't making sense anymore, and then you realise your IDE imported java.awt.List

Even better if your IDE replaces multiple java.awt imports with java.awt.* and you then use a list and start to wonder why it doesn't like your generics. And only after you check that your java version is actually above 1.5 you realize that it was that fucking import all the time.