Pre-Existing GL Error

I was running a simple program and I got this error:

GLDrawableHelper.reshape: pre-exisiting GL error 0x502

    I know that error code 0x502 means that there was an invalid gl operation, but what does "pre-existing GL Error" mean? Has anyone else come across this problem? Thanks in advance.

I got it figured out. I just needed to put -Djogl.debug.DebugGL into the VM arguments to see what the actual error was. Turns out I was setting the vertex shader source as the fragment shader source.