File this one under "completely insane." People are saying that part of why Selena Gomez and Justin Bieber split is because she cheated on him ... with rapper Gucci Mane.

In the words of Liz Lemon, what the what?

Media Takeout (via Hollywood Life) reports that Gomez got "hot and heavy" with the rapper, who was featured in her movie 'Spring Breakers.' Dude, seriously?

A production assistant on set of the movie said that Gucci spent a lot of time with Gomez on set, but we suspect that it may be because, well, they were working together. But apparently Gucci hung out in her trailer a lot, too. “We thought it was [odd] that she was in there with [Gucci]. One time [producers] sent security to make sure she was OK.” The source added, “It almost became a joke, [another employee] said [to Gucci] ‘Watch out or I’m telling Justin [Bieber].’ [Gucci] was like ‘F--- that lil n----.”

Hollywood Life goes on to point out that Gomez wore a Gucci dress to one of the 'Spring Breakers' premieres. Guys, there's reaching and then there's Stretch Armstrong reaching. This sounds completely and totally bogus to us -- almost as nuts as Biebs hooking up with Rihanna.

Watch the 'Spring Breakers' Trailer