There was a race condition in the bullet regression test when compiled in Debug mode. With help from Brian and valgrind, we identified two bugs:
The private bool gazebo::World::stop was not initialized in the constructor, which led to the race condition that we saw, in which the physics engine wasn't always starting.
valgrind identified an increment of an uninitialized pointer in gazebo::Master::RunOnce. The variable iter was cleared at the end of this->RemoveConnection, and then was being incremented right after that function call.