just happened to me with an opponent moving his piece back to square he just left and I reported the bug.I see the same bug was also reported by pauloaguia back in august 09.Check it out.Whats going on here,this will undoubtedly cause someone to lose/win a game they shouldn't at some point
http://brainking.com/en/BugTracker
In fact 4 instances of same bug reported as far back as 2005.Slack job Fencer