Turns out this was a PoPy bug. In particular, PoPy 2.0.2 has a global semaphore it blocks all queries on, including commit and begin. This means if you try to execute a Postgres query that has to block on another transaction (1), you get deadlock, and a hung Zope. -Randy 1: A common case for this is concurrent updates when run in serializable transaction mode (the PoPy way).