The Vancouver Columbian reports the ballots had been scanned but not counted.
The discovery nearly doubles the votes that remain to be counted in an election that includes two races in the 17th District that have been too close to call.
As of Friday, Republican incumbent Don Benton was 96 votes ahead of Democrat Tim Probst in the 17th District Senate race. Meanwhile, 17th District House candidate Monica Stonier, a Democrat, led Republican Julie Olson by 100 votes.
Clark County Auditor Greg Kimsey says the problem was discovered when it became evident that the number of voters given credit for voting did not match the number of affidavit envelopes received.