Northern Ireland captain Aaron Hughes said a lapse of concentration had led to Poland's late winner in Wednesday's World Cup qualifier.
"Our set-piece went straight into their keeper's hands and they broke quickly, won the corner and got the goal," said the Newcastle defender.
"We just switched off and at this level, you just can't do that."
Goalkeeper Maik Taylor added: "We are bitterly disappointed to have lost the game so late."
Taylor said that "tired legs" had contributed to Maciej Zurawski's 85th minute goal.
"We wanted to show everybody that there's a lot of character in the side after the second half performance against England.
"I think we showed that tonight against a top-drawer side but the boys will be very down about the result."
Northern Ireland manager Lawrie Sanchez was left "frustrated" by the late blow.
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It's been a disappointing 10 days summed up by the last 10 minutes
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"We had them on the rack at times, caused them problems, had good claims for penalties and defended stoutly when we had to," he said.
"I'm disappointed for the fans and players - we put in a good 90 minutes and showed we are as good as Poland.
"But ultimately we were undone on the break from our own corner, and to say the least that's a bit frustrating."
Sanchez acknowledged that his side's qualification hopes are now over.
"We're not going to catch Poland now. What we need to do is finish as high as we can in our little group of Azerbaijan, Wales and maybe Austria so in the next campaign we come out the hat seeded a bit higher.
"There is plenty to play for but it's been a disappointing 10 days summed up by the last 10 minutes."