Rae Sremmurd Bum-Rushed Onstage at Ottawa Concert

Rae Sremmurd didn't see it coming -- getting bum-rushed onstage by a dude who also never saw the HARD TACKLE that came after his very poor decision.

The incident went down Saturday night at the Ottawa Bluesfest music festival in Canada as the group was in the middle of performing its hit song, "Guatemala." You can see a line of security in front of the stage, but somehow this guy managed to elude them all and make his way onstage.

Almost right away you know it's a bad idea. The crasher at first tries getting a selfie ... but his ass gets rejected, and then comes the blindside tackle. The guy's subdued and the concert seems to go on without a hitch.

When will crashers learn ... it never ends well. Never.