Adam Forshaw saw red as Brentford lost for only the third time under Mark Warburton at Swindon Town on Saturday.
The Bees midfielder was sent-off for two bookings in the space of three second-half minutes to compound a miserable afternoon at the County Ground.
David Button had denied Michael Smith from the penalty spot before the break after Alan McCormack was penalised for bringing down Alex Pritchard, although the Brentford defender accused the winger of diving afterwards.
However, Button's hard work was undone on the stroke of half time when his hospital ball saw Jonathan Douglas caught in possession, and Louis Thompson pounced to score the only goal of the game.
Mark Warburton admitted a moment of indecision cost his side dear as Brentford's promotion hopes took a knock.
He said: “I am very frustrated with the goal we gave away. It was indecision, and we paid the price for it.
“He gave a difficult ball to Dougie. It's disappointing, but we will learn from that. We were in control, and we had enough chances to even it up."
