Why does clean and build not reveal all exceptions?

Sometimes after doing a clean of the whole project it compil开发者_运维技巧es ok. But then at some later point, a file that hasn't changed starts flagging a legimate compilation error. I've seen this occur on multiple projects?

Why does this happen?

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To get a complete project clean you have to delete all the derived data well. Use the Organizer, then select Projects and look for the Delete button on the right.