Why does clean and build not reveal all exceptions?

Question

Sometimes after doing a clean of the whole project it compiles 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?

Answer 1

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.