This mission can be permanently missed in a save file after completing the mission Last Requests, which means that the player should complete Pump-Action Pimp before that mission if they want to achieve 100% completion and unlock all achievements/trophies.
“Some Diablo scumbag has been pimping his skuzzy bitches in my backyard. Go and take care of things for me. If you need a piece go around the back of AmmuNation opposite the subway.”
Claude drops by Luigi's Sex Club 7 looking for more work. Luigi informs him about some Diablos pimping on his turf that he wants dead and tells him to go around the back of Ammu-Nation if he needs a weapon.
Claude picks up a pistol around the back of Ammu-Nation and attacks the Diablo Stallion with the Diablo and the pimp, killing them both.