SAN FRANCISCO (KCBS) – San Francisco Municipal Transportation Agency (SFMTA) spends $25 million annually on Muni operator overtime, but a new audit by the City’s Controller says as much as 40% of that overtime isn’t documented or verified.

Part of the problem, according to the audit, is with Muni’s payroll system, though management is being assigned a portion of the blame, too. Specifically, supervisors are said to be lax about verifying employee overtime when timesheets are submitted. A review of payroll documents also found that five operators were given birthday premium pay twice in a single year.

READ MORE: Fmr. Pinole Police Officer Allegedly Videotaped Himself Having Sex With Minor

“It’s a very rare circumstance,” suggested Paul Rose, spokesman, SFMTA. “But we’re going to learn from these experiences and learn from this audit to improve that.”

READ MORE: UPDATE: Woman Accused of Starting Fawn Fire Was Boiling Bear Urine to Drink

For his part, Mayor Ed Lee suggested Muni’s oversight of some details may have been the unfortunate consequence of a system that’s trying to improve service for riders.

MORE NEWS: SF Police Arrest 2 Men Who Allegedly Shot Woman Who Refused To Give Them Her Camera

(Copyright 2013 by CBS San Francisco. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)