Oh hey so I found something interesting in v24 - when lights overlap, you occasionally get zones of the map that are 'dark'. Anything within that area will have what appears to be an inverted specular map (if you're using specular maps) and even with light shining on it it'll stay dark.
Obvs not a problem if you keep your lights from overlapping, but I thought I'd mention this in case it's not expected behaviour
EDIT - so it appears to be when two light sources touch a single polygon or mesh, that polygon or mesh doesn't light up correctly. Just gotta be careful with lighting I guess ;-)