Billy Davies' side arrived at Vicarage Road on the back of three wins and three clean sheets and took an early lead through Andy Reid.
But if anyone was going to be the first player to score against Forest this season then McGugan, who hit 44 goals - many of them spectacular - during seven years in Nottingham before a summer move to Watford, was that man.
Forest had gone ahead after only six minutes when Watford's renowned passing game proved their undoing as Marco Cassetti's extravagent crossfield ball was intercepted by Reid.
The Irishman still had plenty to do, first slipping the ball to Jamie Mackie who cut inside before nudging it back to Reid, whose trusty left foot arrowed a fierce drive past Manuel Almunia and into the top corner from 15 yards.
Forest came agonisingly close to doubling their lead just before half-time when Reid's clever pass put Henri Lansbury in behind the Watford defence.
The ex-Hornets midfielder's shot flew across Almunia and crashed against the foot of the far post, with the rebound just eluding Mackie.
But in the 54th minute Watford finally breached Forest's backline. Diego Fabbrini was fouled by Adlene Guedioura 25 yards out and McGugan stepped up to bend his free-kick round the wall and into the top corner.
Both sides had chances to win it with Almunia saving well from Mackie and Karl Darlow denying Sean Murray and Ikechi Anya.
A point was enough to send Forest back to the summit but they almost stole all three at the death when sub Ishmael Miller broke clear only to curl his effort against the post.