Nottingham Forest defender Luke Chambers says their defending must improve otherwise they will blow any chance of reaching the Premier League.
Forest moved into the last Championship play-off spot by beating Leicester City and Bristol City 3-2 over Easter.
But despite their two-point lead over closest rivals Millwall and Burnley, Chambers said losing a two-goal lead against the Robins was "horrendous".
"We cannot keep conceding two and three goals. It will cost us," Chambers said.
The Reds have just two games left in the regular season and could clinch a play-off spot on Saturday if they beat Scunthorpe and other results go their way.
But Chambers was particularly upset with the standard of his side's defending in the nervy 3-2 success over Bristol City at Ashton Gate on Saturday and warned that a repeat could be fatal to their promotion chances.
He added: "What are we doing to ourselves? We were cruising at half-time and played some great stuff and got a 2-0 lead.
"And then whatever the gaffer told us, some of the boys haven't listened basically. Taking a two-goal lead and giving it away so cheaply within 15 minutes of the second half is horrendous.
"We've won the game but we have to look at ourselves a bit deeper and think 'where's that coming from?'
The 25-year centre-half, who has been a key figure in a dependable Forest rearguard this term.
Chambers has started 41 of the 44 league games, with one substitute appearance and centre-back partner Wes Morgan has been an ever-present in the league..
But recent displays have concerned him.
"It drives me insane every week," Chambers said. "If we draw 0-0 then I've done my job. That's what the defenders get paid for and what we take pride in.
"To keep conceding the goals like we've been conceding is so frustrating and that's why I keep losing my voice at the end of every game.
"In the last four or five six weeks we have been conceding a lot of goals. We have to stop that because we have been winning 1-0 and 2-1. Up until the end of January I don't think we conceded more than two goals in a game.
"We've managed to nick a late winner but it should never have been like that. That's why it's so frustrating. We looked so solid at 2-0."
However Chambers also knows his side are close to securing their play-off spot.
"Thankfully we got the points we needed [last weekend], a few teams slipped up and if we win next week, you never know, it could be all done," he said.
"It's a massive game next weekend and we have to be taking three points."
Chambers was also adamant that Saturday's visitors Scunthorpe must not be underestimated - despite a lowly league position and a seemingly doomed fight against relegation to League One.
"I've seen teams that have had nothing to play for, like against Bristol City," he said.
"We've gone 2-0 up and their manager probably gave them a roasting at half-time and they got two goals against us so we can't take any team lightly.
"No matter what position they are in, no matter where they will be playing their football next season and what they are looking forward to we are a scalp for teams in this league. Scunthorpe beat us once this season and we definitely owe them one."