I get the above error when I modify one of my classes that is in a custom palette in the UI designer. I have my project JDK set to be 1.5.0 beta 2. I can get around the error by switching to an earlier JDK, compiling with that, then switching back to 1.5, but that's pretty annoying. I guess this is because IDEA is compiled with a 1.4.x JDK and so the UI designer can't load a class compiled with 1.5? That seems like a serious limitation, basically means you couldn't use a custom palette if you're using 1.5, which would be a silly limitation. There must be a better way around this?