Libpq PQExecParams returns null

Hello

I am using 8.1.3 on FC4

Occasionally a query will fail. PQExecParams returns null and
PQerrorMessage returns an empty string. Subsequent calls succeed.

I found this from a previous post where PQexecPrepared return null.

"AFAIR the only reasons for a null result (instead of a possibly-error
PGresult) would be out-of-memory or connection failure."

I my case subsequent calls succeed and the application is not out of
memory.

What are the possibilities where PQExecParams returns null and
PQerrorMessage returns an empty string?