Old, but notable: herbal supplements may contain cow glands and organs. Something vegetarians might not think about.
Another example of the importance of reading labels: did you know that worchestshire sauce has anchovies? However, I found a number of “vegetarian” recipes on the web that had worchestshire sauce in them.
Robert also pointed out to me last night that Bar-B-Q Baked Lay’s have chicken fat. In fact, a number of Frito-Lay products have animal products in them that wouldn’t be immediately apparent.