I’ve got about a dozen open projects and a similar number of closed projects. Most of them use pretty much the same set of user libraries. SOMETIMES one of the projects seems to forget where its libraries are. The libraries still show up in the build path, but source errors are detected on every import statement for a forgotten library and code references to classes contained in the library. If a library disappears from one project, it remains in others. Sometimes I can clear up the problem by doing a clean on the project. Other times I have to close the IDE, reopen it and clean the project. In all cases, the most recent build had been error-free. Doing the clean feels a little like burning chicken feathers–I don’t know that it always works, but it’s a ritual I can do hoping it will help.
I have no idea what provokes this behavior and therefore cannot reproduce it. It is VERY annoying!