I ran into the oddest thing when trying to debug a networking issue with OpenStack. I was doing a ping test between virtual machines. The ICMP request packets only made it to the destination host if I did a "tcpdump" on the bridge interface on the node running nova-network, which forwards the packets. If I stopped doing a tcpdump on that interface, I no longer saw the requests reach the host.
In general, how/why would doing a tcpdump on a Linux bridge interface affect whether packets get forwarded?