The most common Rock Paper Scissors prank is also the most elegant: challenge someone to a game, let them win the first two rounds, then when they're relaxed and confident, read their behavioral tendency and beat them in round three for the stakes that actually matter. This isn't cheating — it's applied psychology. Losers shift predictably. You're just paying attention.
The second category is social misdirection. Announce that you're "calling" Scissors before the throw — making it clear what you're about to play — and then throw Rock. This sounds like you'd lose, but a significant number of people will throw Scissors to beat what you said you'd throw, and you beat them with Rock. The meta-game of declaring a throw only works once per person, but once is usually enough.
The third type is the "delayed reveal." In casual play where the simultaneous reveal isn't strictly enforced, you can slightly delay showing your hand until you see the edge of their throw, then adjust. This is actually considered a violation in competitive play — referees exist partly to prevent it — but in casual settings most people have done it without thinking of it as a prank. It works because people reveal on different timing and most throws are detectable by the shape of the hand even partially formed.
What doesn't work: any prank that requires modifying the rules after the game starts. "Actually I was playing Dragon, which beats everything" produces an argument rather than a laugh. The humor in RPS pranks comes from working within the structure of the game — exploiting human predictability — not from arbitrarily changing the rules. The game's structure is simple enough that any attempt to hack it from the outside is obvious immediately.
The most durable RPS prank isn't a trick at all: it's just getting very good at reading people and then acting surprised when you keep winning. After five or six consecutive wins, the other person starts to wonder if the game is fixed. It isn't. You're just watching their hands.