Reproduce in the codes-net repo as follows:

make tests
cd tests
mpiexec -n 2 ./concurrent-msg-recv --sync=3 --codes-config=conf/concurrent_msg_recv.conf

ROSS patch to address the problem is attached. The fix is to prevent the network code from bypassing scheduling in the fast case if the cancel queue isn't empty. Otherwise re-issued forward events may bypass cancellation events, causing the model to see two identical forward events without a rollback in between them. This can cause problems for models of stateful protocols.