When the spider jumps and then goes into a different sector with a different height, thus interrupting the jump code and the spider reverting back to the walk cycle before it lands. That one. :3

Ah that one. This has already been fixed somehow and it's included in the last wad uploaded. Here's a map that's been setup for this kind of test if you want to give it a try:

Oh, whoops. Must have missed that one or something lol. In this case this demon seems to be fully complete and all fixed up as far as I can see.
Thank you for all your help BlueShadow. I really appreciate it. :3