I've gotten that sometimes with old objects as well (sometimes they're green!) They aren't objects that I've converted personally, but have downloaded here around the forums. I think they may have been Cosmic Prophets, but I don't remember. Anyway, I've never bothered to see what was causing that because first of all I was just playing around and secondly they seemed to work okay upon launching a test. I'll see if I can recreate it and I'll get back to you!
EDIT: LOL, it might be a few days!
MAME Cab PC: i7 4770@3.4Ghz (passmark 9945), 12GB RAM, Win 8.1/64, GeForce GTX645 (passmark 1898); Shiny new laptop: i7 4800MQ@2.7Ghz (passmark 8586), 16GB RAM, Win 8.1/64, GeForce GTX870M (passmark 3598); Old laptop: i5@2.3Ghz, 8GB RAM, Win 7/64, Intel 3000 graphics