Frustrated Brentford boss Uwe Rosler admitted the opening goal was crucial after his side's 2-0 defeat by Leyton Orient.
The Bees matched the runaway leaders for 65 minutes until Shaleum Logan fouled David Mooney in the area.
Mooney stepped up to fire in the penalty and the O's never looked back, with substitute Shaun Batt wrapping up their eighth straight win late on.
Rosler said: "There were a lot of positives but all the time we say we can't afford to let in the first goal and we keep doing it.
"I was disappointed with our reaction after Orient scored when we kept going long instead of trying to play ourselves in with patience. That was very disappointing, but I am not going to slate my players.
"I told them we were looking strong in the first half and to keep doing the right things, but you can't afford to make a mistake like we did for the penalty against a good side like Orient."