Crash in Release mode after extended time

I have a ragdoll system working, and an editor where you can assign capsules/boxes/spheres to bones and link them together with joints. It works well, and when I create a full ragdoll skeleton and "turn it on" the mesh falls down appropriately. However, if I leave the simulation running for 5 minutes it will crash on exit with one of those "send / don't send" errors. The logs seem to indicate that nothing gets shut down when one of these crashes occurs. It only happens in release mode (debug mode works cleanly) and I don't have any memory leaks.

Any suggestions on how to go about fixing this?