I ran into a momentarily scary bug in the NetBeans 7.2 IDE as I was working on a Java program. I'm posting details here in case anyone else is doing what I did -- desperately googling for a fix.
I had just successfully run my code from the IDE and committed the changes (using Git, although I doubt that's relevant). Somewhere between doing the commit and possibly starting to edit a file -- I forget exactly where -- the IDE gave me a low memory warning. So I exited and restarted the IDE, thinking that might free up some memory. When I went to run my code (unmodified from the previous successful run), the program would not start. Instead, I was informed in the terminal window that a ClassNotFoundException had occurred. The named class was definitely there, at least in source form. I tried a "clean and build" to recompile the code, but the exception repeated. Restarting the IDE (again) did not help. Rebooting the PC did not help.
So I went off to Google to hunt for the reason for the exception, and found a variety of bug reports (from 2011 onward) that might or might not be pertinent. Fortunately, one of them contained a suggestion to turn off the Compile on Save feature. You can access this, which is on by default (at least for me), by right-clicking the project in the Projects window and clicking Properties > Build > Compiling. I turned it off, and sure enough the program would again run from the IDE. So I tried a "clean and build", verified the program ran, switched Compile on Save back on, and the bug returned.
Compile on Save does what is sounds like: it compiles source files when you save them. This saves time when you run or debug the program, so I was not entirely thrilled at having to turn it off. Before submitting a bug report, I did a quick search of the NetBeans issue tracker, and found only one bug report (again, from 2011) that mentioned both Compile on Save and ClassNotFoundException. The person who filed that report resolved his problem by deleting the NetBeans cache (which NetBeans rebuilds when next run). So I tracked down the cache (on my Linux Mint PC, it's in the folder ~/.cache/netbeans/7.2), deleted it, restarted NetBeans, turned Compile on Save back on, and happily discovered that the program once again runs.