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Don Lemon had another "open mouth, insert foot" moment on CNN ... this time using the word "obviously" to describe the smell of marijuana in the air during the protests in Ferguson.

Lemon was on the scene Monday night as residents took to the streets to protest the decision not to indict the police officer who shot and killed Michael Brown.

It kinda turned into quien es mas macho between Lemon and Chris Cuomo. Lemon had gas mask in hand and Chris went commando.

At one point as gas moved in Don's direction he put the mask on but for some reason took it off, saying he needed water and started hacking uncontrollably.

Sad part for Don and Chris though -- Sara Sidner outmanned both of 'em by taking a rock to the dome ... and not missing a beat.