Serbia had turned 0-1 to 3-1 when Cameroon created new nerve in the game in the second half - thanks in large part to Vincent Aboubakar.
The striker was substituted in the 55th minute and just nine minutes later he made it 2-3.
With the smallest possible margin, Aboubakar was on the right side of the offside line and deliciously chipped in the reduction.
Two minutes later came the equaliser.
After a counterattack, Aboubakar found his way to Choupo-Motingo who could easily make it 3-3.
The headlines before the match were otherwise about Cameroon's goalkeeper André Onana, who was poked for disciplinary reasons.
Substitute Devis Epassy was put to the test straight away and Aleksandar Mitrovic threatened the most.
The Serbian star had an early header over the crossbar and then hit the post after eleven minutes of play.
Nice wall play on the right wing opened up Mitrovic who made a move into the lane before getting a good shot off with his left foot.
Despite Serbia owning much of the game in the first half, it was Cameroon who took the lead.
A header on a corner reached Castelletto who, completely unmarked at the far post, was able to score 1-0 after 29 minutes.
However, two goals in extra time saw Serbia go into the break leading 2-1 and also made it 3-1 at the start of the second half before Cameroon began their recovery.
Cameroon completes the group stage against Brazil and Serbia against Switzerland.