Kettering manager Mark Cooper has been left frustrated by his side's 2-1 defeat at Stevenage in the Blue Square Premier on Tuesday.
The Poppies went ahead through Jason Lee, only for the hosts to equalise and then claim a winner with just six minutes to go.
"To be honest it's unbelievable that we didn't get a result there," Cooper told BBC Radio Northampton.
"The game was there for the taking and two mistakes have cost us."
He added: "But if you're serious about achieving things, you can't keep on making those mistakes."
It was a second setback for Kettering in the space of four days, following their late FA Cup exit against Fulham at the A-Line Arena on Saturday.
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