A WAIT statement specified an event variable. The completion of the event caused entry to an ON-unit for an I/O condition which contained another WAIT statement for the same event variable as in the original WAIT statement.

DCL F FILE
RECORD OUTPUT UNBUFFERED
ENV(BLKSIZE(80) RECSIZE(80) F);
ON RECORD(F) BEGIN;
WAIT(E);
END;
WRITE FILE(F) FROM (X) EVENT(E);
WAIT(E);  (this statement raises the
record condition)