Originally Posted by ChrisPlatt 
C41 films (and all negative films in general) are far more tolerant (i.e. have a greater latitude) for overexposure rather than underexposure.
It is easier to print an image from a too-dense negative than from a thin negative - you can't print what's not there.
As mentioned underexposure will also lead to noticeable color shifts, too.
Chris
That is the conventional wisdom to be sure. What some of the earlier posters have been saying is that there appears to be less latitude for overexposure in one or more layers. Whether this is loss of reciprocity, localized exhaustion of developer, or an interaction with the dye binders is anyone's guess. A little bit of controlled testing is probably called for here.
Steve
(My thinking at this point is that we are seeing a localized exhaustion of developer during processing...helpful in B&W work, probably not as desirable with color.)