McGugan, who left the City Ground for Watford in the summer, curled in a superb free-kick to cancel out Andy Reid's opener and secure a 1-1 draw.
A point was still enough to send Forest back to the top of the Sky Bet Championship while preserving both teams' unbeaten starts to the season.
But one unnamed Forest player will be getting a earful for his part in the equaliser.
"It was a good advert for the Championship, we're pleased with how we played and attacked, but we are disappointed with the goal," said Kelly.
"If the right person had been on the right end of the wall it might not have gone in!
"But it's a good point against a good team who will be up there challenging this season. It says something when you come to a ground like this and are disappointed you haven't won."
- Sports & Recreation