Apollo 6 (2nd Satun V launch) was "less than nominal" and warranted a congressional hearing. It did succeed, but luck played a part. George Mueller declared later that Apollo 6 was a failure for NASA.

https://web.archive.org/web/20080120112115/http://www.hq.nas...

https://web.archive.org/web/20080227133401/http://www.hq.nas...