After not running JBuilder 8 for a bit, when I tried to rebuild an applet at $work, I received a very strange set of missing class errors. These classes were right there in the Jar libraries... I swear :)
Well after a few hours of looking around I found that I need to remove the whole 'output path' option (under Run -> Configurations -> Path tab). Then it magically worked.
I know JBuilder8 is ancient but one applet builds in it and nobody wants to fund it moving to Eclipse...