TWO sending offs and a 97th-minute goal condemned the Reds to defeat in a ferocious South Yorkshire derby against Sheffield Wednesday on Saturday.
Just 24 hours after joining from Arsenal, Emmanuel Frimpong was shown a yellow card twice in five first-half minutes.
Wednesday substitute Jermaine Johnson followed him on 80 minutes while Barnsley midfielder Jacob Mellis later on.
The dismissals proved costly for Barnsley as Maguire drilled home from 20 yards - seven minutes into injury time.
Despite the hosts' strong start, the first chance of note fell to Barnsley striker Nick Proschwitz who was twice denied from a corner, first seeing his 12-yard volley blocked on the line by Liam Palmer, before Chris Kirkland saved the rebound.
On 23 minutes - after being warned by referee Andre Marriner for persistent fouls - debutant Frimpong was finally booked.
It didn't take long for him to pick up a second yellow when, five minutes later, he brought down Maguire with a clumsy challenge from behind.
The home side grew agitated, culminating in a straight red for Jermaine Johnson after a two-footed challenge on Brek Shea.
Looking destined for a goalless draw, another twist happened soon after when Mellis recklessly jumped in on Wednesday winger Jacques Maghoma and was sent off.
And it was Barnsley who were made to pay when substitute Jim O'Brien miscued a clearance in the 97th minute straight to Jose Semedo.
He fed Maguire, who provided the day's one moment of real quality to settle an ill-tempered South Yorkshire derby.
Boss Danny Wilson said: "I feel for the team as a whole because we were brilliant when we had 10 men.
"Jacob's red card was just stupidity but I think it was frustration from the stonewall penalty being turned down. We could have won the game but instead we've lost it."
Next up for the Reds is a home game against Ipswich Town on Saturday.