Saracens director of rugby Mark McCall described the decision to rule out Marcelo Bosch's try just before half-time as a “game-changer” as they saw their LV= Cup hopes dashed by Northampton at Franklin's Gardens.
With an easy conversion to come it would have seen Saracens take a 14-6 lead, but television match official (TMO) Derek Bevan ruled Nils Mordt's pass to Jackson Wray in the build-up as forward.
McCall said it was "marginal" as Northampton came back in the second half with a George Pisi hat-trick sealing a 26-7 win and booking their place in next Sunday's final at Sandy Park, their seventh final in seven years.
McCall said: "It was a great try so it is always disappointing and it was a game-changer, it would have been 14-6 for us at the time.
"We will look back on what might have been.
"For the first 50 minutes we played really well and had three quite big opportunities but only converted one of them and against a team like Northampton you have got to take those chances. They were very clinical, when they got into our 22 they always came away with something and the game just got away from us."
It was Northampton's 13th win in a row in all competitions, a record run for the club in the top flight and a fourth successive win over Saracens.
Northampton director of rugby Jim Mallinder felt the winning streak his side is on helped them come through an enthralling semi-final.
"Saracens could have easily scored another couple of tries, but we managed to keep them out and in the second half we improved," said Mallinder.
"Momentum is important, it gives you belief and the players know how to win games. We were under the cosh in the first half and it was brilliant to come out in the second half, believing we could win it and ultimately doing it."
On whether he felt it was a forward pass for Saracens' disallowed try, Mallinder added: "It is hard to say, but the crowd normally get it right here!
"We were a little bit rocked losing Tom Stephenson early on. Tom has been our defensive captain for the last few weeks and having to make that change disrupted us a little bit.
"It gave Saracens the opportunity to get on the front foot, which they did, but we re-grouped in the second half and defensively we became a lot stronger."