Undo sometimes doesn't show up

[vinay]

Sometimes pressing "undo" does nothing - it doesn't pop up the "waiting to see if the other player will allow the undo" box for the player who pressed undo, or the "allow the other player to undo" box for the other player.

An example game where this happened is game #6258842. I think Possession messed it up there, maybe. I've seen this happen a number of times.