The Guardian reports that some of our favorite food and drink, including Guinness stout, contain unusual animal products. The frothy dark brew, for example, is "clarified" using a form of collagen called isinglass, derived from the swimbladders of fish. ( Remember the "isinglass curtains you can roll right down, in case there's a change in the weather" from the song " Surrey with the Fringe on Top," from the musical Oklahoma!?? Not the same--that isinglass apparently is made of mica, and was a substitute for glass.) Only trace elements of the stuff remain after brewing, but that may be enough for rejection by staunch veggies.
Mars Bars candy, which once used the vegetable version of rennet in its milk chocolates, sometimes made from fig leaves or thistles , or mold, is back to the animal variety, derived from calves, or "the stomach(s) of slaughtered newly-born calves," as the UK Vegetarian Society so nicely puts it. Rennet allows the calves to digest their mothers' milk.
As for beef gelatine? Evidently it's in Kellogg's Frosted Flakes, at least in UK products, to help the sugar adhere to the flakes.
( Photo from http://www.guinness.com/us_en)