ClickToFlash (http://clicktoflash.com/) is a very popular utility for Mac OS X/Safari that allows you to control what Flash movies are loaded in pages.
I fiddled quite a bit with it, but couldn't get the Glitch sign up process to work with it installed, even after adding alpha.glitch.com, glitch.com and glitch.bz to the exceptions list. I ended up uninstalling ClickToFlash in order to get to sign up for Glitch.
I don't think this is a bug, so I am sticking in the Ideas area.