Did the Nets give this away with a third-quarter defensive collapse, or with a couple of curious coaching calls in the endgame of overtime?

Probably both. There was plenty of blame to go around in Monday’s galling 96-95 overtime loss to Miami before a sellout crowd of 17,893 at Barclays Center.

It wasn’t decided until Mikal Bridges (team-high 26 points to go along with nine rebounds and six assists) saw his baseline fadeaway against three defenders fall just short with 1.1 seconds left in overtime.

But so much had to go wrong to even get to that point.

Brooklyn (16-23) had to blow a 16-point third-quarter lead, its defense imploding in a Jimmy Butler parade to the free throw line.

After missing seven straight games with a toe injury, Butler returned with a game-high 31 points, 15 of 16 from the charity stripe.

That was encapsulated by Dennis Smith Jr. getting whistled for a foul with 11 seconds left in OT, a call that coach Jacque Vaughn opted not to challenge and that handed the Heat star the go-ahead free throws.

Bridges’ miss made Butler’s final foul shot the game-winner.

“I don’t think I fouled him. But the decision to challenge, that’s not my decision. Whatever we go with, that’s what I’m rocking with. Whatever the team is going for. So it is what it is. I feel like I play good defense, though,” Smith said.

“I got a chance to look at it real quickly during the game and just felt they weren’t going to overturn that,” Vaughn said. “We felt Dennis had his left arm wrapped around the backside of the driver, and I didn’t think using the timeout they were going to overturn that call.”

And after the fact, Vaughn opted to let it ride, not calling a timeout to let the Nets attack.

They got the ball in the hands of the man they wanted, but Bridges missed the last look.

“We have shooters on the floor, great opportunity for them not to sub,” Vaughn said. “They kept [Tyler] Herro in the game. We tried to attack him and turn the corner. Really good defense by Jimmy Butler. We got a shooter spotted up in the left corner, Royce [O’Neale] is peeling at the top and Cam Thomas is over in the right corner.

“I don’t think I’m gonna be able to draw anything up better than that. Nic [Claxton] is in the dunker ready to offensive rebound. So the pretty clear for us … good defense by them.”

Good enough to send the Nets to their 13th loss in 16 games.

“I didn’t get fouled,” Bridges said. “They made shots and got physical, and that’s when the run started.

“We were getting stops, but you can’t let making shots control our energy. That fight, that’s what it gets to.”

Bridges didn’t get fouled; but what was foul was having to stomach blowing a 47-31 lead in the third quarter and a five-point edge in overtime.

The Nets allowed the Heat just 26.2 percent shooting — and 0 of 12 from deep — in the first half.

But Brooklyn surrendered 70.6 percent — and 5-for-7 from 3-point range — while getting outscored 37-24 in the third quarter.

The fourth quarter couldn’t settle the game — Bridges knotting it at 88 with 4.4 seconds left in regulation — so they needed overtime.

That’s where the Nets blew yet another lead.

Thomas (23 points off the bench) put the Nets on the board first, and after a Bam Adebayo miss, O’Neale’s right-wing 3-pointer gave them a five-point lead.

A Bridges block on Caleb Martin led to Brooklyn forcing a shot clock violation.

Herro (29 points) hit a 3-pointer with 1:29 left, but Bridges got a bounce on a floater in the lane for a 95-91 lead.

Another Herro 3-pointer pulled Miami within one.

And after Brooklyn was called for offensive basket interference, Butler earned a whistle against Smith, hitting both free throws.

He gave the Heat the lead and Bridges’ miss ensured they kept it.

“We just wanted whoever got it, me or Mikal, just get downhill to try and get the best shot we can,” Thomas said. “That was the best shot he could’ve got, I guess. We just have to live with that.”