Silent ORA-904 on MERGE

From time to time looking at the tkprof’ed SQL traces I see this message:

The following statement encountered a error during parse:

SELECT 1 FROM DUAL WHERE some_predicate_here

Error encountered: ORA-00904

A some_predicate_here is the predicate from ON clause of a MERGE statement executed by the session. Up until recently I didn’t know why that happens, and since the error is not reported to the end-user there’s no big problem other than very little annoyance.