Reece Marshall made an eventful return from injury as the Wanderers lost 29-26 to London Irish in Monday night's Franklin's Gardens friendly.
The hooker managed to write his name on the scoresheet, but he was later sin-binned, with Irish taking advantage to score a winning try, through scrum-half Rory Brand.
The Wanderers had gone 26-17 up during the second period, but Irish fought back superbly to snatch the win.
It was a fiercely-contested game at the Gardens as Irish, who stepped in at late notice to provide the opposition after Wasps pulled out, showcased their power and class.
Visiting fly-half Theo Brophy Clews was pulling the strings before he was forced off with an injury picked up creating a try for former Clermont Auvergne wing Napolioni Nalaga.
And the Irish pack went toe to toe with the Wanderers, giving them a real test.
The home side still looked likely to emerged victorious thanks to tries from Matt Worley, Marshall, Fraser Strachan and Ollie Sleightholme.
But Irish kept fighting and after Nalaga scored his second try of the night, Irish made the most of Marshall's sin-binning to score the winner.
It was a fine win for the away team, who were up against the likes of Tom Wood, who got 60 minutes on his return from injury, Api Ratuniyarawa and James Grayson.
The Wanderers had gone in front almost immediately as a high ball caused confusion and wing Worley picked up the pieces to race in from inside his own half.
Grayson added the extras with a tidy conversion, but London Irish responded really well, putting pressure on their hosts at maul time.
Eventually the Wanderers cracked as hooker Ben Atkins burrowed over, but Brophy Clews was unable to add the extras.
However, London Irish continued to impress, defending well when they had to and using the ball to good effect when they got their hands on it.
Wing Ciaran Hearn finished off a flowing move out wide to give the away side the lead, but Brophy Clews, who had set up the score with a fine pass, missed the conversion chance.
Irish had a 10-7 lead after showing little fear during the formative stages of the game and James Grayson missed a difficult drop goal chance to level the scores.
Reece Marshall, on his return from injury, was doing his best to drive the Wanderers on, making some eye-catching hits to stop Irish in their tracks.
The Wanderers managed to get within range from a lineout drive and Marshall helped himself to a try on his comeback, with Grayson converting to make it 14-10 at the break.
Wood moved to No.8 for the early stages of the second half, while Marshall picked up where he left off, flattening an opposition player within seconds of the restart.
But London Irish continued to impress and they scored first in the second half as some lovely work from Brophy Clews opened the door for Nalaga to score.
Brophy Clews suffered an injury in the build-up, leaving replacement fly-half Jacob Atkins with the duty of landing the conversion, which he did.
But the Wanderers weren't behind for long as centre Strachan, who had been impressing in defence, found his way over the line after a spell of concerted pressure.
Grayson converted and there was another Wanderers try almost immediately as Tom Emery released Sleightholme on the left and the player wearing 13 ran in for the score.
Grayson missed the conversion but the Wanderers now had the comfort of a 26-17 lead with 25 minutes to go.
But the breathing space didn't last for long as Nalaga nabbed his second score of the night and Atkins' conversion cut the gap to just two points.
And Irish were soon at the Wanderers' door again, with their forward pressure forcing Marshall to infringe, earning himself a sin-binning that ended his eventful evening.
It wasn't long before the Wanderers were made to pay as a big Irish scrum gave their No.9, Brand, a chance to dot down with ease.
Atkins missed the conversion, but the Wanderers were now 29-26 down with the clock against them.
And despite a couple of lineout chances deep in Irish territory, the Wanderers could not get back in front as Irish celebrated a characterful victory.
Wanderers: Furbank; Worley (Evans 77), Sleightholme, Strachan, Emery (Dayes 77); Grayson, Tupai (Creighton 77); Davis, Marshall, van Wyk (Garside 40); Moon, Ratuniyarawa (Onojaife 40); Wood (Ma'asi 60), Ryan, Eadie (Wallace 40).
London Irish: Loader; Hearn, Fowlie, Mulchrone, Nalaga; Brophy Clews, Brand; Chawatama, B Atkins, Saulo; Twomey, De Chaves; Gilsenan, Cooke, Curtis-Harris.
Replacements: Frampton, Baker, Collingridge, Englefield, Moore-Aiono, Hassell-Collins, J Atkins.
Referee: George Richardson