This latest defeat, the fifth from seven league matches this season has left Forest fourth from bottom of League One and puts Megson under severe pressure to keep hold of his job. The Forest faithful certainly showed their fustrations and Barnsley deservedly pocketed three points, which moves Andy Ritchie's side closer to the top six.
Ironically it was Forest who had the edge for the opening 15 minutes despite losing top scorer David Johnson with a shoulder injury. His replacement , Eugene Dadi, offerned a more physical threat but Barnsley were probably more pleased with that than having to contend with Johnson's pace.
After 15 minute Forest almost opened the scoring when Scott Dobie met a right-wing corner with a firm header from eight yards but home keeper Nick Colgan produced a magnificent save to prevent the ball from crossing the line.
From that point onwards Barnsley took control and began to find their way though the Forest defence. Top scorer Jacob Burns, a driving force from midfield, twice went close with 25-yarders while Chris Shuker saw a low shot well pushed away by Forest keeper Rune Pedersen.
Barnsley finally made the breakthrough in the 42nd minute when Pedersen could only half clear a free-kick. The ball fell to Shuker who's drive was going away from goal but ran kindly to striker Barry Conlon who stabbed home from eight yards.
Dobie sent an overhead-kick just wide a minute later while a brilliant cross on the run from Gino Padula deserved better but there was no Forest player on the end of it.
To make matters worse for the visitors Barnsley bagged a killer second goal in first-half stoppage time.
Shuker cleverly exchanged passes with Paul Hayes and from the left hand side of the box, riffled and unstoppable shot across Pedersen and into the far corner.
Megson changed formation at the start of the second half going from 3-5-2 to 4-4-2, but it made little difference although their best player, Kris Commons, was unlucky with a fierce drive that clipped the outside of the post.
But Barnsley defended stoutly, with Matt Carbon proving head and shoulders above the Forest forwards and it was a welcome three points that maintains Barnsley impressive start to the season..