With some browsers (and with some gateway/hosts) there is a problem with caching. Sometimes you will select a move and it will go back to a previous instance of this move instead of going forward and updating your record and getting the correct result. If this happens, hit the Reload button on your browser and this should execute the move properly.
Update:
As of 10/28, this should no longer be a problem except, perhaps, with circles.
If you believe that you have a valid bug report, send email to friedman@cs.stanford.edu.