With the match seemingly over, Old Boys knocked the ball on from the kick-off, giving Marist a scrum 40m out.
A slightly controversial penalty for collapsing the scrum was then called by debutant referee Simon Miller, giving Marist the chance to snatch victory.
Marist first-five Mariu Grace calmly slotted the penalty.
Marist coach Dave Palmer
paid tribute to the performances of second-five Tuhulu Isileli-Fo'ou and openside flanker Hayden McCaw who had a typically busy day. Palmer said performances from Kyle Aaapa and Northland's Dan Faleafa were integral in the win.
Old Boys coach Boyd Matson said it was unfortunate his team could not continue their winning form against Marist but credited wing Daniel Cotton, prop Morgan Tohu and centre Troy Gilbert for their efforts on Saturday.
In other club rugby results, Kamo beat Hora Hora 36-12 at Hora Hora, and Western Sharks snuck home by a point over Mid Northern, 29-28, while Otamatea were too strong for Hikurangi in winning 29-18.
Wellsford were handed their first defeat of the season by Mid Western, who prevailed 17-3.
Wellsford led 3-nil for almost the entire first half only to have their hard work undone by a sustained Mid Western attack on their line.
Mid Western maintained the ball for a number of phases before releasing the ball wide for winger Ben Mathers to score just before the break, giving them the lead 7-3.
With Lachie Munro unavailable, Mid Western called upon halfback and skipper Adam Clarke to fill the void at first-five. Clarke's control played a part of his team's second try, which was similar to their first.
After another sustained period of attack, Wellsford's line broke as new Canterbury recruit Shaun Hill crossed the line. Hill had played 80 minutes for the premier reserve team before being substituted on due to an injury after four minutes in the premier game - earning him player of the day in both games.
Mid Western's third try came late in the game after a line out which turned into a rolling maul, pushing Wellsford back 10m and giving prop Pete Mirrielees a rare chance in the spotlight.
Whilst Clarke controlled the game well, Mid Western did lack the goal-kicking finesse of Munro.
Mid Western coach Simon Cauty said it was pleasing to give Wellsford their first defeat of the season, in the process leap-frogging them on the points table to sit second.
"It was our defensive effort that was the key in the end. Not to let them score a try was really pleasing, especially after they started strong in the first five minutes.
"They scored an early penalty, but after that we sort of took over and dominated the game."
Cauty said his team will stay focused throughout the competition with it being one where most teams are capable of beating each other.
Next week's round sees Hikurangi take on Hora Hora at home, Kamo vs Western Sharks, Marist host Mid Northern, Wellsford at home against Otamatea, Old Boys will host Mid Western.