Im doing this on purpose, just to see what happens. The result i really would like to achieve is something similar to this ; http://www.flickr.com/photos/8746365@N02/2361989729/ but that's probably wishful thinking. I'll post the results when the film is processed.
You could probably achieve something similar with color neg by just underexposing 3 stops and pushing it. That is a loose reciprocation of how age affected film behaves.
It has color shifts and may become fogged. But modern films can stand an awful lot of abuse. I've abused exposed film very badly in closed cars in the desert southwest, and it's always turned out OK. Maybe there is a bit of deterioration (not often), but it is always corretable and the negatives or transparencies have always been quite printable.