Admittedly, you see some cards more than other cards, so there's some things everyone is up to date on (If two people have a Consecrated Sphinx, then it's pretty much draw till you are bored) but with something like 15,000 different cards ever printed, and such a small Banned list, there's always going to be something you haven't seen before.
Me, I enjoy losing in unexpected ways. I've lost in an Unhinged draft, when I was about to attack for the win, because I touched my Vile Bile. (for about two days after that match, after he'd lost a lot of life to that card, Mark wouldn't touch new cards without reading them first)
So I've compiled a list of cards that are capable of winning the game out of nowhere. We're not talking "Time Stretch, copy it, dig for my combo and I win 20 minutes later" or "Elf makes Elf makes Elf" but one card that ends the game, sometimes as part of an attack, sometimes in response to an attack.
Some of these cards need other cards, or other situations, but they are capable of sending someone on that roller coaster of "I'm about to win!" to "Oh bleep, did I just lose?" in the space of one card being put on the table.
I'm going to start with an obvious and semi-obnoxious one, and move on from there.
This is a card that was a standard centerpiece for a long time. Decks would use the Urza lands (Mine, Tower, Power Plant) and Reap and Sow to build up and up and cast this. Tutor up Darksteel Colossus and Kiki-Jiki, or Sundering Titan, or whichever fatties the situation called for.
Commander is both better and worse for assembling foolishness. It's a little harder to ramp as fast in this format, but there is a ridiculous number of fatties to go find. A new favorite is Avenger of Zendikar and Craterhoof Behemoth, so that all your creatures, and the Behemoth, get the bonus of the number of lands you have.
Generally speaking, Tooth ends the game for one or more at the table. In a Riku deck, expect that someone is going to find Kiki-Jiki and Pestermite for infinite attacking creatures.
Now, the less obvious cards:
Delirium/Backlash/Boros Fury-Shield/Mirror Strike
These cards do similar things. They take an opponent's creature and turn it against that player.
Seems innocent enough, except that Commander is a format where enormous creatures can be seen on a regular basis and are therefore ripe for abuse.
If someone has a big enough creature, that doesn't have hexproof/shroud, then they are dead to these cards. Each of these cards has certain limitations, but they are unexpected.
Delirium is only good on someone else's turn, while Backlash has to target an untapped creature. If they somehow tap it in response to your spell, then Backlash is countered. Mirror Strike requires it to be attacking, and not blocked--not easy to do unless you tell the attacked person not to block!
As fun as it is to see someone get domed by their own enormous Hamletback Goliath (an M13 reprint, so it will be more common this summer) or other huge creature, Delirium, Backlash, and Mirror Strike work with Blightsteel Colossus ridiculously well.
Because the creature deals damage, not the spell, infect is relevant. You may know someone with an infinite lifegain combo, and if they have a BSC in play, it's game over for them!
It's also worth noting that three of these cards are red/white and cost less than four mana, meaning that they are excellent Sunforger targets--which is not the first time you'll read those words in this post.
This card, which is excellent in BW decks, is NOT a good Sunforger target. You won't pay any mana for casting this spell, so neither effect happens, which is why Boros Fury-Shield is not a good Sunforger target either.
Aside from that problem, this card is a house against token decks. Did someone just Tooth and Nail for Kiki-Jiki and Pestermite? Did they attack the table with an arbitrarily large number of creatures?
Lose an arbitrarily high amount of life, buddy.
Once your friends know you have this card, always leave BW conspicuously open, and watch their faces contort as they try to figure if you have the card or not for the threatened GOTCHA!
This is another card that just punishes token decks. We all know that person with the Rhys the Redeemed deck, or some other shenangians where they need a pound of counters to keep track of everything.
Mark Rosewater has a Sliver Queen EDH deck that this card would be awesome against. Saw it played at Worlds.
It's should be pointed out that transmute is a pretty great ability on a six-drop card. Again, great ability, and utility when that ability isn't needed.
Another great Red/White card for when someone is trying to end a game. This card is a catchall against many styles of deck, but it works on massive spells and huge creatures alike.
Blasphemous Act, you say? Go home, sir.
Banefire for 50? Nice try.
Magma Phoenix died? So did you.
This is one of my favorite plays in response to Congregate.
Note that this works on all players for the rest of the turn, so if you have some crazy 'everyone gains life' play (ahem, Arbiter of Knollridge) then yeah, you can use this as an offensive card instead of a reactive "GOTCHA!" moment.
Keep in mind that if you do this in response to an effect, they can wait on doing that effect again until the next turn. To use this well, it has to be a lot of lifegain on the stack, like Congregate or they played Avenger of Zendikar with a Soul Warden out.
White/Blue doesn't have an Overrun effect, but it does have this.
The 'nonlegendary' clause means that you can't use this as an instant-speed Wrath of God, but you can turn a few creatures into a few awesome copies!
Before or after blocks, this card is capable of some extreme blowouts. Use with Thiseldown Liege for some incredibly difficult counting.
And if you're Red/White/Blue (America!) then yup, Sunforger away.
By far, this is one of my favorite Commander cards. You get to change what one person wanted to do AND do it again!
This can double up on lethal spells, and because your copy will resolve first, can even win the game in the face of something like an Exsanguinate for 40!
Red card, 4 mana....yes, yet another amazing Sunforger target.
This pair of cards allow for creatures to become much more powerful than they were, and at instant speed.
On one hand, it's a way to make big hits into lethal hits, which is what you want your creatures to do--beat face.
On the other hand, it can be used in conflicts that don't concern you, and that can lead to some really annoyed people.
Fun Fact: You can use either of these cards with the foursome of "Creature hurts its controller" cards if you have the mana in the same turn! "End of turn, give your creature infect, then Delirium it!" GOTCHA!
This final card is not always going to be helpful, but it's tricky in a way that Black never has access to.
It costs a lot of life, and only works against single targeted spells. Willbender is better in such cases, as he gets effects and can be re-used with a bounce effect.
Still, the Mischief is capable of ruining someone's plans, and if it costs you some life, you still get a great "GOTCHA!" moment.
This is not the end-all, be-all of awesome tricksy plays. these are just a few of the cards that can reverse a game state very quickly.
Feel free to tell your own "GOTCHA!" stories in the comments.
And go enjoy M13!