If you look at the Trophy Taker Full Containment rest, it can actually overextend. There's a small washer that goes between the rest arm and the rest frame on the draw cord. If that washer is missing, and you pull the rest ALL the way back, it arcs over center, and drops back down just slightly at full draw. I have had this happen to me on my FC400. All it took was a little adjustment and some serving right above where the drawcord goes through my cable. Now it stops right at the high point of the arc, and not past that point.
I took a closer look at my Apache rest, and noticed that it can't do this though. It stops well short of full vertical. However, if you have the rest set to not come completely up at full draw normally, it would be possible for a twist in the cord or the anchor point of the cord moving to cause the rest to come up higher than normal.