O.J. Simpson's controversial prank show was supposed to be a joke ... but production wasn't funny -- in fact, his costar says he got really angry at one point.

Investigation Discovery is releasing a new docuseries titled "O.J. Unseen: Exploitation of Evil" this week ... which chronicles the making of "Juiced" -- a pay-per-view prank program released in 2006.

If you haven't seen it... O.J. pretends to try and sell a white Bronco at a car lot, dresses up as an Elvis impersonator, and pretends to have an affair with a man's GF -- basically, a typical prank show of the era. However, one skit seemingly went awry ... with some actual tension bursting to the surface.

Check out this teaser from the doc ... which featured Todd Lewis -- the director and producer of the ill-fated project -- with whom O.J. gets into an argument.

O.J. calls Todd racist in the clip ... a move Todd says went too far and really upset him. Todd says he didn't want to upset O.J., fearing a wrong move might set the dude off ... but eventually the two were getting into it.

Todd says he looked at O.J. at one point and says he "saw anger in his eyes," and he genuinely didn't know what might happen next.

Worth noting ... "Juiced" came out in 2006 -- and O.J. was arrested and charged with armed robbery and kidnapping the following year.

"O.J. Unseen: Exploitation of Evil" premieres on Monday, September 7, on ID ... and will be available to stream the following day on HBO Max.