In Season 2 of Fringe (spoiler):
Charlie Francis finds himself alone in a hospital boiler room with a shapeshifter in the form of a nurse. The shapeshifter kills Francis and takes his form. When the other agents arrive, he is standing over the body of the shapeshifter/nurse and the shifter device is in the nurse corpse!
Assuming that the shapeshifter had plenty of time to change the bodies, why he left the shifter device in the nurse corpse?
It makes no sense, he is stuck on Francis body because of this! Even if (somehow) the device is broken, why leave it there? It makes no sense to help his opponents by providing them this device for them to study.