iRule events firing out of order?

Just to reassure you (and other readers), it is impossible for iRule events to fire out of order, though this is mostly true by definition. An iRule event fires whenever that action happens, so SERVER_CONNECTED fires whenever we have successfully connected to the server. That said, I don't know what could be causing the problem you describe. The only thing that makes any sense to me is if the client connection closes after we get the SYN-ACK from the server and before server data arrives. That would sever the relationship between the client and server connections, and I think it just happens that variables are associated with the client connection. You could test this theory by testing inside SERVER_DATA if the variable exists, and if it doesn't then see if there's a client connection (maybe do a catch of clientside { [IP::client_addr] } and log the result, or something like that). Just a thought.