Chaz Mostert was left to rue early-race contact with Will Davison, which triggered a penalty.
Davison and Mostert were disputing sixth in the wet OTR SuperSprint finale.
The latter nudged Davison at Turn 6, sending the #17 Shell V-Power Ford into a spin.
Davison was lucky to avoid being stranded in the wet gravel, but Mostert wasn’t so lucky.
Officials deemed it a driving infringement, and Mostert served a 15-second penalty at his compulsory stop.
To make matters worse, Davison managed to finish fourth despite spinning again.
It was a sour end to an otherwise strong weekend for Mostert, who finished on the podium in the first two races.
"Overall, pretty positive weekend,” he said.
"To get two podiums was a great result and one we will continue to build off.
"Last race was a little disappointing, I’m pretty dirty on myself for bumping into Will.
“Sorry to him, because the car was good.
“To get inside the 10 after the penalty, you can’t complain with that I guess.”
Davison was pleased to fight back after the skirmish, finishing eight seconds from the podium.
"The wet race was really exciting with crazy conditions,” he said.
"I really enjoyed it, I got a great start and we were really in with a good shot to challenge for the top couple of places.
"I got spun early and then dropped right back. Then I was obviously attacking really hard and we were super fast.
"We had a few costly mistakes trying to charge back through the field and the conditions were changing all the time.
"We ended up in fourth and it was a good salvage and recovery and a lot of fun.”
Davison and Mostert are fourth and fifth in the points respectively heading to Sandown.