In Maniac Mansion (V2/DOS/English} with ScummVM SVN 1.0.0pre043561 (August 20, 2009 22:34:52) on Windows XP, purple tentacle shows up in the lab, stuck after Weird Ed chases him away. This happened after a cutscene with Dr. Fred and Purple Tentacle came up, after purple tentacle was already gone.
After that, he's stuck in the middle of the lab, but you can't interact with him.
Save game of purple tentacle stuck in the middle of the lab
Still occurs with latest Git master and attached savegame:
ScummVM 1.4.0git2591-g153e663-dirty (Oct 5 2011 20:45:39)