If you upload a broken plug-in which causes an exception when loading, it does the following:

a) Prevents all other 'dropped-in' plug-ins from loading
b) Prevents removal of the offending plug-in.

To fix the problem, you have to stop your web server, delete the offending jar, and restart the webserver. Not fun, particularly when developing a new plug-in, where errors happen semi-frequently. Plus, it doesn't give very good error messages. The one I'm getting at the moment is 'java.lang.NullPointerException'. No stack trace - just the exception classname.