I have a league game against drsteelhammer that might help locate the bug.
Scheme was included in game 3276955. On my last turn, I wanted to undo a Masquerade play. dsh granted, but it kept pending on my side. dsh then resigned.
After reloading at the last decision point the log (and deck contents) were different than before. (e.g. I was supposed to have trashed an Encampment, and dsh had bought a Moat, which both never happened in the original game). That was game 3277403. (I think the log was already off after dsh resigned the first game)
We then reloaded at the earlier decision point 150 (now game 3277461), and everything was fine (other than the last turn obviously missing). We didn't try any more undos.
Another (probable) bug is that both of the loaded games show up in "Scavenger", even though they were selected to be unrated (and loaded games shouldn't be rated). And I suspect that they will also be rated in the official leaderboard.