Nelly is looking back at his short-lived rap beef with Eminem with a facepalm ... signals got crossed and he jumped the gun while on MTV like a jackass!!!

The "Country Grammar" rap star recently told Bootleg Kev he misinterpreted something Eminem had said and went on 'TRL' and set off the battle -- quipping how he eats "M&M's candy" and St. Louis was a no-fly zone for the Detroit hip hop legend!!!

Em fired back, casually ripping Nelly on the mic with a line about "Stomping in my Air Force Ones."

Nelly apologized profusely to Eminem -- not because he was shook but because he realized he was barking up the wrong tree when he met Em backstage at one of his concert after-parties, and Em's daughter Hailie Jade was his biggest fan!!!

Nelly says the moment made him remember he and the St. Lunatics were big Eminem fans ... and whatever tension they had quickly dissolved.

Eminem had plenty of other rap beefs popping at the time that demanded his attention, and Nelly went on to push his sophomore album into the multi-platinum stratosphere.