Had this one in a formal assessment, student lost 30 min before we could recover his code. This is the minimal set of steps to reproduce the error:
new project, new class Test, new class Test2, let Test somehow use Test2, compile, all well.
open Test in editor, type "import static Test2;" as first line - does not compile, no reaction.
close editor, click compile in main window - errors are shown in editor for Test.
close editor, close project, try to open again - the UML diagram is completely broken, project cannot be opened.
It is not severe, as the Java code does not make sense and so the error will likely not occur again in student code too soon. But the reaction of BlueJ is scary.