Clicking on it and cancelling the log in dropdown seems to trigger the problem - it may take several attempts to trigger it.
Once it fails to work, clicking on it repeatedly in the same place does nothing.
However, moving the mouse fractionally results in a working button, until the next time it fails.
Chrome 32/Win 7
Edit: Video demonstrating the problem: http://screencast-o-matic.com/watch/c2nbnbnIun