Sometimes, when I attack, the game grabs the top unit of the stack and attacks with it alone. This has happened three times, with stacks of two units. The remaining unit is then visible but un-selectable until I refresh the browser completely. It most recently happened in my new game, forcing my cavalry to attack one at a time, and sacrificing both of them to a neutral city that should have easily been conquered. I ignored it as a possible user error until it repeated itself a couple of times in separate games.
It's not a simple double-clicking issue, as I am left with a bugged unit on the map each time.
Using:
Windows Vista (ugh)
Firefox, recently updated
I updated to the brand-new Firefox version just about two weeks ago, whatever it is. So that's what I have been using.