The phenonomon Evilmike describes can also be seen by loading groundforce levels in single player.

I have seen another error as well, where jj2 will crash (1.23 is the only version I have seen this happen, and only on two single-player levels) with an memory error if it tries to load levels that have, as the error describes, "Too many enemies or animations".

Strangely enough, loading and launching the level through JCS will load the level with no problems.