San Mateo County election officials said Thursday that 65 uncounted provisional ballots from November's election were found April 12 in a vault at the department's headquarters.
According to election officials, the ballots were found in the vault inside a covered bin that had prevented them from being seen.
The 65 ballots, which came from the Redwood City voting center, included 35 from eligible voters, with the other 30 coming from people who were ineligible or had already voted in the election.
The number of uncounted ballots did not change the outcome of any contests or measures, so the county's election results remain certified, said San Mateo County chief elections officer Mark Church.
"This is an unfortunate and completely unacceptable incident. There is simply no excuse for this type of error in the elections process," Mr. Church said in a statement.
"The importance and significance of each vote cannot be understated. It is the cornerstone of our democracy," he said. "Proper steps will be taken to avoid incidents of this nature from ever occurring again."
The provisional ballots will remain sealed in their envelopes and will be destroyed after 22 months, in accordance with election laws, according to the department.