I get those as well from time to time. Bug in Eclipse, I believe.
Check out the content assist bug list. If nothing there is relevant to your specific situation you can open a bug.
Yuval A
2009-03-28 18:14:28
A:
Its been happening consistently now, don't seem to be going away
Click Upvote
2009-03-28 18:18:11
+2
A:
Any Eclipse errors should be logged, so see if you can see them in the Error Log view. Open it either by pressing Ctrl+Shift+Q, L or using the menu Window > Show View > Error Log.
As Yuval A suggests, either find the existing bug (and read any workarounds) or report a new one in bugzilla - you'll be helping out Eclipse developers and users (not to mention yourself) by reporting the problem.
McDowell
2009-03-28 19:01:30