[ BUILD # : 6.5 Beta ]
[ JDK VERSION : 1.6.0_05 ]

1) In a new Java Application project, create a class FooBar and use
it in the class Main.

2) Open a java editor on the Main.java file.

3) Delete the file Foobar.java

See the first problem: nothing is red-underlined in the view of the
Main.java file. (see screenshot 1)

4) Close the Main.java editor tab.

5) Reopen it, you will normally see the errors as underlined stuffs.

6) Restore your Foobar.java file to where it was before.

Second problem: the underlined stuffs stay there. (see screenshot 2)