Chris D'Elia's really testing whether a joke can be "too soon" ... turning a terrifying moment when a man started seizing in the front row of his comedy show into a 13-minute-long bit.

The comedian just posted a clip to his Instagram showing his performance at Levity Live in Oxnard, CA last weekend, when the scary health incident went down ... kicking off when the man tipped back in his chair, and Chris verbally checked on him.

D'Elia asks him a couple times if he's OK, and while the guy gives him the affirmative, he quickly passes out again ... and it's clear he's in distress.

A couple guys carry him away from the stage to get him medical attention ... and Chris jokes his bit about Pride Month must've been too much for the guy ... which draws a few gasps, prompting one audience member to say the joke's coming way too soon.

Chris takes off from there ... saying he's performing at a comedy club, so what the heck else is he supposed to do -- a dramatic monologue???

This escalates into a full set from the comedian on this incident alone ... running nearly 15 minutes long about the audience and their reaction to the alarming experience. He also chats with some coworkers of the dude who needed to be taken away, and they say he's going to be fine.

We reached out to Chris ... and, he says this has gotta be one of the wildest moments that's ever happened during one of his performances -- and, because it happened so early, he had to keep the show going for another 40 minutes.

He says the staff called an ambulance and get the guest the medical attention he needed ... and, he's got nothing but praise for the Levity Live crew for handling the situation.

BTW ... Chris says he's happy to give free tickets to the guy who had the seizure -- and, all he's gotta do is hit D'Elia up.