I’ve noticed a bug that shows up quite often (almost every session) on Mac OSX 10.7.
IIRC this never happened in Snow Leopard, it’s only occurred since I’ve updated to 10.7 and is still present in 10.7.2. I’ve read that Apple updated OpenCL and it has caused issues for other Mac users, but I personally have only seen it in Renoise. If it helps it seems that this appears when I pause playback, although it’s also happened by just moving the mouse.
I personally have never seen this on the mac I use renoise on most frequently - os X 10.5.7 …
ahh. 10.7… that’s likely the issue. 
What kind of video card are you using? Dual link DVI cable? have you tried zapping your pram and maybe trashing the renoise prefs??
Ah, you must have signatures hidden, I have a MacBook Pro 7,1 from 2010 with a GeForce 320M. Just using the built in screen so no DVI cables. Haven’t tried to fixed honestly, it just hasn’t really bothered me. I’ll try resetting my pram and deleting the preferences.