I have a strange rendering issue with Mental Ray in 2009. *gasp!*
It took me ages to narrow down the problem- but it finally turned out to be "film offset" attribute of my camera- if it's not 0 then I start to get strange illumination artifacts in my renders- I've attached a couple of renders to demonstrate.
It only happens when I have any sort of a texture (file) plugged into the light source. Anybody else had this freaky issue? I'm rendering stereoscopic- and it's rather difficult to work around not using film offset.