I am using Pitbull, although I dont think this is a pitbull problem...I only get this when in a group and in combat...( I read on wow forums that someone was getting this even w/o add ons...so...any ideas guys?
Does it happen consistently when you're grouped an in combat? If so, you can very easily find out if PitBull or another addon is causing the problem by logging in without PitBull enabled, join a group, enter combat, and do whatever triggers the error. If it still happens, it's not PitBull. Repeat with no addons at all enabled. If it still happens, it's not an addon. If it doesn't happen when no addons are enabled, start enabling addons until it happens again, and you'll have found the problem.
I'm having the same problem but you really can't know when the error will come i did try everything to fix it but nothing working so far. some people say it is blizzard mistake and it will be fixed in 3.0.3 .
This also solved the same issue that was happening for me. I would really like it if I could have the threat meter turned on though, it's handy (as I am a tank). Is this just the target frame having an issue, so if I used a different unit frame for that would it solve the problem?
Rather than switching away from unit frames you're familiar with and happy with, you could use Omen3 instead of the Blizzard threat meter. Omen now uses the Blizzard API functions to get threat information, so it no longer incurs all of the CPU overhead and addon comm bandwidth overhead that it had to pre-3.0.