They recovered to trail 31-24 with two more tries, then after conceeding a penalty to open the second half, Wanganui began to shut the hosts down and chipped away at the scoreline, getting a converted try and penalty, while giving away another three-pointer for Mid Canterbury to be clinging on 37-34 with time running out.
Mid Canterbury had lost flanker Seta Koroitamana to the sin-bin and Wanganui's sixth try with around eight minutes remaining seemed to confirm a famous comeback as they finally led 39-37 and got right back into Mid Canterbury's half.
However, a penalty only 5m from the tryline gave the Cantabrians the chance to clear and then another infringement at the breakdown allowed Percival - who plays fullback more than No10 for the province - to take his shot at glory.
For Jason Hamlin, it was a match that had come down to "coach killer" moments.
"It was a tale of two halves," he said.
"We dropped the ball on the kickoff and they scored a try within a minute.
"There was an intercept try where they went the length of the field and they got in on a cross kick.
"We maintained a bit of possession in the second half and we eventually broke them down and led.
"Recurring theme, our disciple and decision-making at crucial times let us down.
"It's nothing about the referee - we have to play with him and adapt to the situation."
Back in the starting lineup, winger Cameron Crowley scored twice, as the backs contributed all the tries with second-five Timoci Seruwalu, centre Kaveni Dabenaise and fullback Jona Sawailau crossing, as did first-five Craig Clare as part of a 14-point return.
Mid Canterbury fullback Maleli Sau scored twice to stay one try ahead of Seruwalu on the 2017 leaders list.
With news that the injured Fraser Hammond is now likely gone for the season, while Campbell Hart needs at least two more weeks, flanker Tremaine Gilbert continues to carry the can in the loose forwards.
Hamlin was also impressed with the reserves coming on during the comeback, as halfback Tom Stewart, the returning Nick Harding and especially Collegiate teenager Cody Hemi all did well.
"We just got to decide if those guys are better at status quo - coming off the bench - or do we make the changes."
Young lock Cade Robinson also did his best in his first 15 minutes of Heartland rugby.
As cruel as a 'miracle' kick can be to snatch defeat from the jaws of victory, Wanganui had ultimately left themselves vulnerable after conceeding four first half tries, as Hamlin said they showed a "lack of communication" with the man-on-man defence.
"[Mid Canterbury] were able to play with a lot of width and just got outside us.
"We didn't really adjust.
"In the second half, we did a better job and shut them down. But we keep digging ourselves a hole."
It was not the way hooker and captain Cole Baldwin had hoped to celebrate his 100th first class match.
"We did enough to win the game at the end, but it wasn't to be. Tough to swallow," he said.
"Just disappointed we got ourselves in that situation.
"Give my boys a bit of credit, they did well to fight back."
Mid Canterbury 40 (Maleli Sau 2, Willie McGoon, Christian Vainerere tries; Jarred Percival 4 pen, 4 con) bt Wanganui 39 (Cameron Crowley 2, Jona Sawailau, Timoci Seruwalu, Kaveni Dabenaise, Craig Clare tries; Clare pen, 3 con). HT: 31-24.
