It could be easily done I think - it apparently only effects the people with it, so it might modify some of the memory values, maybe some stuff in the JJ2exe, who knows. But it couldn't come from a level or tileset file, it is most likely from an exe (almost certainly). So maybe it came in those little programs and trainers and all people like fiddling with while in single player mode. But this does seem like a serious problem.

Hm...if you can't get rid of it, don't just reinstall. Uninstall JJ2, delete the extra files left over in the folder, and go download and run the JJ2 Registry Key remover so you can reinstall JJ2: http://www.jazzjackrabbit.com

I THINK that's what you might need to try anyway, although you should ask someone like Alberto, Kaz or Newspaz first. I'm not going to be held responsible if you uninstall it wrong.