After playing for a while on 4-6 things started to get a bit glitchy like missing tiles and projectiles not showing up. Not sure what triggered it.
<img src="http://dl.dropbox.com/u/9299424/hmm.png" border="0" />
Chrome Version 24.0.1312.57 m
This has only happened once to me while developing it. If you restart the browser and load your game it will work fine again.
I don't think it's a bug I can do anything about, it's probably a browser issue. :)
Kudos for your progress in the game - it's a hard and long game!