It's been mentioned in another thread already, but IIRC some of the weighing agent they mix into the plastic is metalic. This can cause a DG to get flagged going through X-Ray for carry-ons. It's happened to be a couple times now where they ask me if any of these things are metal.
Add in the fact that you have a big black bag with a big skull on the side, and things are about to get interesting.