The party comes across a magic amulet. This amulet will give each user two wishes, but the wishes almost seem to have a mind of their own.
The encounter here is each of the wishes themselves. Each wish is a puzzle. In order to get your wish granted, you must make a declaration with the words "I Wish" in it. If you make one by accident, it still comes true and the wish is wasted (or worse, the wish is harmful).
Even more importantly, a wish must be contained within a single sentence, and wishes with too many clauses in them, can be arbitrarily shortened at a reasonable point (where a sentence might naturally end). For example: "I Wish I had a sword wreathed in cold, that freezes my enemies when it hits them" might get reduced to "I Wish I had a sword wreathed in cold".
The trick is to keep it short but specific. "I wish that I could hover" is too ambiguous. "I wish that I could fly with the maneuverability of a humming bird", is both short (notice that there are no commas) and clear. Also, it should be obvious, but metagame terminology just won’t work here. Players can’t wish for a +5 sword. They also can’t wish to gain a level, or gain an ability (at least not in those words).
What the user doesn’t know, is that there is a cap to how powerful a wish can be. Wishes that are too powerful will get shortened, misinterpreted, or just plain fail.
For example "I Wish I was a god" could cause all of the pebbles within a mile to gain sentience (but not animation) and worship him as a deity. "I wish for a Ruby the size of a mountain" might get shortened into "I wish for a Ruby". As a GM, it is important to set this cap before the session starts.
The goal with this encounter is not to try to skew everything out of context. A player wishing for a sword could have that sword appear lodged in the player’s heart. They could also get a sword that is very very small, but either would defeat the purpose of the encounter, and ruin any fun for the players.
With that said, there is a certain amount of free license that is expected. Creativity is a big part of the scenario, and pouncing on particularly bad phrasing is part of the encounter. Not all bad wishes have to go spectacularly wrong though, and a wasted wish can still produce a good effect. For example, a player who wishes for a stone, (meaning to wish for a gemstone) could have the unexpected surprise of having a rock that follows them around (rolling magically after them).
Expanding the scenario
1) Only one wish per player.
GM’s who want to be hard on players can give them only the one shot with the amulet. Preferable for larger parties.
2) The players don’t know the item grants wishes.
Any wishful thinking on a players part comes true. It’s up to them to determine why weird stuff keeps happening.
3) The Amulet needs coaxing.
The Amulet is sentient, and will be particularly difficult about interpreting wishes unless the players befriend it first. Befriending it may involve simply taking care of it for a while (keeping it polished) or completing some kind of quest.
The Amulet may also start off liking or disliking certain characters. The fighter may have a hard time getting the amulet to like him, while the thief may have a particularly easy time.
4) The wishes favor certain requests.
Certain types of wishes are favored over others. Wishes that are selfish in nature may be very difficult, while wishes that help others go surprisingly well.
Favored wishes may even add benefits that weren’t wished for.
5) Wishes have a random clause.
Wishes all have an additional random clause added to them that changes the intent of the wish. "I wish for a sword wreathed in flame" might turn into "I wish for a sword wreathed in flame, between the sheets". Everybody will be surprised when the player’s pants catch on fire.
Some examples are:
"Between the sheets"
"Like a good bra"
"Strong enough for a man, but made for a woman"
"Built for a giant"
"Like a Rock"
6) The Amulet doesn’t always remember how the wish was phrased.
The Amulet is fallible like a person is. Each time a player makes a wish, another player has to repeat what was wished for, without help of any kind. The paraphrased wish is used in place of the first one.