Oxford United caretaker-manager Mickey Lewis had no excuses after his team crashed 3-0 at leaders Chesterfield, had two players sent off and ended the game with eight men.
An eighth-minute strike from Ollie Banks after a mistake by Dave Hunt opened the door for Chesterfield and the gap got wider in the 16th minute when Deane Smalley needlessly barged over Ian Evatt and Gary Roberts dispatched the penalty.
Oxford regrouped in the second half and might have got back in the game if Ryan Williams had scored when he went clear, but home goalkeeper Tommy Lee raced off his line to make his second vital save of the game.
Tom Newey was red-carded for a crude lunge on Banks in the 80th minute and when Michael Raynes was shown a second yellow card for tripping Roberts three minutes later, the game was up.
Oxford were left with only eight men on the field when Johnny Mullins was stretchered off and Eoin Doyle released Roberts in stoppage time to chip Ryan Clarke for his second.
The visiting fans chanted "we want a manager", but Lewis said: "It was nothing to do with formation, shape or personnel, we gave two awful goals away to put ourselves on the back foot.
"We should have dealt with the first at the back post and then Deane has given a really stupid penalty away.
"When you are playing top of the league and you're 2-0 down after 15 minutes, it's always going to be difficult.
"Second half we stayed in the game and if Ryan Williams goes through and scores it's 2-1 with 25 minutes to go, but then the game's dead when the two lads get sent off.
"Perhaps frustration set in, but there are no complaints about the sendings off
"We are giving far too many goals away and just have to get back on the training ground and keep working."
Chesterfield assistant manager Leam Richardson said his side were good value for their win.
He said: "The result is fantastic and I think we deserved it.
"There are a lot of incidents to talk about, but I think the correct ones have been taken and we will just look after ourselves.
"Scoring early blows their game-plan out of the water, they have to come out and gaps are left which we could expose.
"I think this was back to our best going forward at times and we were quite dynamic.
"The manager doesn't hide from the fact he wants to play open, expansive football and try and win every single game.
"So it has been frustrating when teams don't come and have a game so to speak, but teams are allowed to manage and play how they want."