Just ask Collin Sexton -- who learned the hard way that Tristan is deadly serious when it comes to breakfast pastries and rookie hazing.

Here's the deal ... Sexton only grabbed 12 treats instead of 24 which pissed Tristan off ... and for this, his $50,000 ride paid the price.

"There are 12 popcorn bags in your vehicle which represent the number of doughnuts you have failed to provide on game days," a sign taped to his whip said.

"We require 2 dozen doughnuts on game days, not a dozen."

It's obvious Tristan played a role in the popcorn assault ... 'cause dude threw the vid on social media and said, "That boy going to find out today. Bring enough donuts, or this is what happens."

"Respect will be had! Duties will be done!"

For his part ... Ben Simmons -- who Sexton played against Sunday -- didn't think the punishment fit the crime, saying, "Man, why they do you like that, man? That's crazy."