Just observed in game #107798222
I spent a favor for the Woodworker's Guild and was asked to trash a card. I clicked "Don't trash" and the game stuck. Only way out was to request an undo.
Thanks for reporting, saavedro!
Yes, this is a bug. Technically speaking, the option to not trash an Action shouldn't be there. Once you spend a Favor, you have to trash an Action if you have one in hand.
I'll forward this.