Charlton head coach Michael Appleton felt referee Thomas Kirk had “a shocker” for Cambridge’s dramatic late equaliser at The Valley.
The Addicks looked to be coasting towards maximum points as they led 2-0 going into the closing stages after second-half goals from Alfie May and Chem Campbell.
Gassan Ahadme sparked hopes of a comeback when he headed home George Thomas’ corner and then the Ipswich loanee converted a penalty in the sixth minute of stoppage time, securing new Cambridge boss Neil Harris a point in his first game in charge.
Kirk awarded the spot-kick despite Tayo Edun’s foul on Jack Lankester appearing to be just outside of the box.
“I’m not going to defend the referee because it is indefensible,” said Appleton.
“Before the lad has even hit the ground the whistle is in his mouth. He has given the penalty and it is clearly outside the box.
“I just told him he had a shocker. Clearly that is what linesmen are there to do – to try and help make decisions. But I don’t think he even gave the linesman an opportunity to make a call or a say on it.
“There are so many mixtures of emotions. The way the first half panned out a lot of players looked a little bit nervous and anxious – they struggled playing with an expectation on them, which is a little bit alarming at half-time.
“Having said that, with the changes we’ve had to make over the last couple of weeks and the players we haven’t got playing at the minute due to injury – the physical presence they have – it wasn’t a massive surprise.
“In the second half we were on the front foot, aggressive and passed the ball forward much more like ourselves. We go 2-0 up and could’ve scored another couple.
“Just little bits of detail have let us down at the end. We can’t allow a free header from seven yards. It is impossible to allow that to happen at any level of football.”
Former Millwall, Cardiff and Gillingham boss Harris signed an 18-month deal with the U’s earlier this week.
“First half we were outstanding and if we’d come in 3-0 up then you’d still feel Charlton got away with that – it could’ve been four or five because we were that good,” said Harris.
“The response to come back from a two-goal deficit was terrific. No-one switched off or downed tools.
“No-one gave up and the subs made an impact. The character, resilience, camaraderie and quality to come back in that game like we did, I can’t praise the players highly enough.
“I’ve always enjoyed friendly banter with Charlton fans. Of course to come from behind like I did, as I did in my first game as Cardiff manager here after being 2-0 down, is quite fitting for me. My first three days in the job have been mega.”