This one I couldn't reproduce 100%, but it happens pretty often not to mention it.
Many times I remove the source of error and the class (the tab and the file in project structure) remains with the error underline. I think it has something to do with class dependencies.
For example, I have a class which is in an erroneous state only because I forgot to add the proper module libraries. After I add them, the class stays error underlined. I need to force a reparse (by modifying it) to remove the error.