I managed it to come to a state where Window → Show View → Other... → Java → JUnit does simply nothing in my Java EE perspective. It behaves as if it's already shown but it isn't (at least not visible to me).
The JUnit view is shown in the Debug perspective as normal.
eclipse.exe -clean doesn't help.
I'd like to avoid resetting the perspective (and, hence, didn't try that, yet).
UPDATE
The same happened with the Package Explorer view right now (a day later). I tried to restore it from the status bar and this time a bug report popup appeared: Widget is disposed. It's invisible now, it's not shown as minimized any longer and it cannot be activated via Window → Show View → Other... → Java → Package Explorer.
Try the right sidebar - it may have got here through accidental clicking !