1) Flash player indicates it is your turn (possibly from hitting flash refresh).

2) You have a set and click trade cards.  Default flash player then indicates that you have cards available for trade, but you have already opened the trade cards window.

3) The flash pop-up that indicated you had a trade-in will not go away, nor can you navigate away from the cards window.

The only way to bypass this that I have found is to reload the page.  This can prove problematic in games with a very fast countdown.  It is also just a nuisance factor, nobody likes freezing/crashing. 

I only experience this in quick games, but I imagine anytime you load up the flash player and click trade-cards in advance of the notification that you have a set to trade in.