Hadn't seen this thread before, my first instinct was the peanut butter (I now know that unopened jars of JIF are suspicious objects, and having 6-10 of them in your bag will cause interesting things to happen).
The one I haven't seen mentioned is leaded crystal. I usually keep this in my carry on bag (and pull out for separate screening), but I forgot on a trip last year and put a large piece in my checked bag (also forgot to pull out the piece that was in my carry on).