Michael Higdon volleyed the winner in the 73rd minute as Motherwell beat Celtic for the first time in exactly two years.
Chris Humphrey had opened the scoring prior to the break, before Georgios Samaras headed an equaliser for the visitors.
James McFadden had come off the bench twice since re-signing for Motherwell, appearing in consecutive defeats against Dundee United and Ross County which had seen Motherwell slump from second to fourth place. But his first start since playing for Everton against Wolves on May 6 last year saw them return to second spot.
Anthony Stokes had Celtic's first chance on the break but he mis-kicked an attempted volley following James Forrest's cross.
The game remained finely poised and Motherwell took the lead in the 31st minute. The goal originated from Darren Randolph's goal-kick with McFadden heading the ball on for Henrik Ojamaa to put Celtic on the back foot.
Ojamaa laid it off to Keith Lasley who threaded the ball into the path of the onrushing Law. The midfielder turned centre-back Victor Wanyama inside and out before aiming a shot to the far corner.
The effort beat Forster but would have gone wide had Humphrey, who was level with Emilio Izaguirre when Law shot, not run in and finished off at the back post from close range.
Joe Ledley replaced Tomas Rogic at the break and Commons had Celtic's first shot on target in the 57th minute, with Randolph comfortably behind it.
An object appeared to be thrown from the Motherwell fans in the East Stand while Commons was waiting for Efe Ambrose to come on for Kayal before taking a throw.
Celtic soon silenced the home support with a goal from nothing in the 63rd minute. Forrest made space to cross from the byline on the right and Samaras seemed to catch out Randolph as he raced in to head inside the near post as the Irish goalkeeper came off his line to anticipate the ball dropping into his hands.
Wanyama was booked for hauling down McFadden, who came off and was replaced by Stuart Carswell in the 71st minute while Gary Hooper replaced Stokes, and Motherwell were back in front two minutes later.
Ambrose and Matthews got in each other's way at a high ball in the right-back position and Law broke down the left again following Ojamaa's pass. Law turned Ambrose and stood up a left-footed cross to the back post where Higdon arrived to cushion a volley into the roof of the net.
Celtic launched men forward looking for an equaliser but Randolph overcame his earlier blip. The goalkeeper saved from Ledley at the near post, caught a Commons header and held Forrest's drive before the home side survived a late aerial onslaught.