The most fun I've had with strace was debugging a 3-process deadlock. An snmp daemon was blocked waiting for a cli child process to finish, the cli was waiting for a response to a message on a socket it had open with a routing protocol daemon, which was waiting for a response from the snmp daemon.

It is also a great way to figure out why programs without useful debug output die. Ie. after a program opens and reads a config file it doesn't like, it starts cleaning up and exits.