Rob Page admitted Northampton only had themselves to blame for a 3-1 defeat at home to Millwall.
Cobblers had to play with 10 men for the third successive league game following the second-half dismissal of Jak McCourt.
They were already trailing to Lee Gregory's first-half goal and fell further behind when Calum Butcher gave Millwall a 2-0 advantage. Substitute Marc Richards reduced the arrears but former Cobbler Steve Morison sunk his old club with a late third goal.
Page said: "Milllwall came here to get a result and we fell into their trap.
"We got back into the game after half-time but the sending-off changed the game and typifies the highs and lows in football. Jak McCourt got us a point at the leaders last weekend but he's made a mistake today.
"He's been fantastic since he came here and he will learn from that. But when you're on a caution and you make a challenge off your feet, you have to win the ball - if not, you're off.
"It's not worth taking the gamble and it's a harsh lesson for Jak to learn.
"That gave us a mountain to climb but the three goals we gave away were embarrasing. Millwall didn't have to work too hard for any of their goals.
"If you defend like that you are not going to give yourselves a chance. We have got to get those errors out of our system because individual mistakes are costing us at the moment.
"We will address that on the training ground on Monday and be ready to go at Peterborough. The players have been really consistent so far so we'll deal with that and move on."
Millwall manager Neil Harris said: "It wasn't pretty at times but it was a typical League One game between two evenly-matches sides.
"Northampton play really well to their strengths and we like to go direct when we need to at times.
"We did the dirty stuff better than they did, especially in the first half. But even with 10 men, Northampton were a threat, they kept putting balls into the box and that was a real test for us.
"But we restricted them to long-range efforts apart from the goal but I'm not going to allow that to overshadow the display. We were very good going forward and they were three very good finishes.
"We've had a bit of a blip recently and I'm pleased for my players, to come here and win is a really good start for the week coming up. I'm delighted we're going back to the New Den on Tuesday, we've got a good record there.
"Today was a good test for what we can expect from Bolton Wanderers. They've got some really good players but we've got to carry this momentum in Tuesday's game."