BB also had a recall not long ago because of vitamin D toxicity. I think MSU discovered it. Animals were getting sick.
Interesting that BB says there are no serious side effects to an overdose of vitamin D. It's a fat soluble vitamin, and as far as I know there can be very serious issues with it.
Blue Buffalo Recall - Vitamin D Michigan State University Diagnostic Center for Population and Animal Health issues statement of concern about vitamin D toxicity associated with diet; Blue Buffalo issues recall Diagnostic Center for Population and Animal Health
Looks like it may have even caused kidney damage in some dogs, so to say there are no serious side effects tells me they either wanted to make it sound like a small issue or know nothing about medicine (or both which is likely).