The Bluebirds were 1-0 up against the London club following Jordon Mutch's opener but they were soon to be pegged back.
By the letter of the law, a foul should have been given against the striker but Taylor allowed the goal to stand.
Mackay told BBC Sport: "At the time, I didn't see but I've been in to see the referee. Our goalkeeper bounced the ball. He saw the player standing there.
"The referee is saying that, at the time, he heard from his linesman that he dropped the ball. He said immediately, if the goalkeeper bounces the ball, it's a foul on him.
"He obviously bounces the ball and it's a huge mistake. A horrendous error. They've got it wrong between them and it's cost us. We've gone in one-each at half-time instead of one-nil."
Cardiff are 16th in the Premier League following their defeat.