I am working with 2 instances of IntelliJ 4.5.4 and one project references a jar that is being developed on the other project. When I build the jar with ant a new method being added was recognized from the other project as reference (shows up on autocomplete) but gives error on compile time. According to sun this is a "bad class version problem". We solved the problem by deleting all *.class files and remaking the jar. Normally Project Rebuild should do this!!!