This is a special case of a DeadLock: two processes both wait for the other to continue. It is relatively easy to produce such a condition f.ex by writing a piping together a program that alternates between writing a line its output and reading one from input with one that expects all input in a single batch and produces all output in a single batch.
See Synchronisation
2 pages link to DeadlyEmbrace:
lib/main.php:944: Notice: PageInfo: Cannot find action page