Another term that is not-infrequently used when discussing message passing application is “unexpected messages.”
What are they, and why are they (usually) bad?
The quick definition is that an unexpected message is one that arrives before a corresponding MPI receive has been posted. In more concrete terms: an MPI process has sent a message to a process that hadn’t yet called some flavor of MPI_RECV to receive the message.
Why is this a Bad Thing?