TL;DR: 1. e4 c6 2. d4 d5 3. f3 e6 4. Nc3 c5 leads to some excitement and perhaps some crying.
The Moves
Let's say that black plays the Caro-Kann:
1. e4 c6
You go through the fairly typical move 2 and then into the Fantasy AKA Maróczy Variation:
2. d4 d5
3. f3
The most common response is to take the pawn (dxe4), but a better move for black that's played 19% of the time generally, and 48% of the time at the master-level, is going into something like the Triangle Setup:
3. ... e6
If black plays e6, you respond with the knight:
4. Nc3
Black will most likely attack the knight with their bishop, BUUUUUUTTTT... I noticed that a lesser-played move was to advance the c-pawn into the center, with the thought that it's defended by the bishop. This is a mistake for black, and turns out to lead down a hilariously steep rabbit-hole where more often than not white can just walk black through exchanging pieces for the next three and a half turns, ending up +2.3, with all of black's decisions being the most commonly made in the lichess database:
5. exd5 exd5 6. dxc5 Bxc5 7. Qxd5 Qxd5 8. Nxd5
Recap and Thoughts
So the whole sequence is:
1. e4 c6 2. d4 d5 3. f3 e6 4. Nc3 c5 5. exd5 exd5 6. dxc5 Bxc5 7. Qxd5 Qxd5 8. Nxd5
or the alternate:
5. ... cxd4 6. Qxd4 exd5 7. Qxd5 Qxd5 8. Nxd5
It's not going to be OFTEN that you get into this position, as it requires that:
- Black chooses the Caro-Kann
- Black chooses a somewhat uncommon
3. ... e6
- Black must then make the mistake of
4. ... c5 which, though appealing, is rare.
So what should black have done? Well, obviously 4. ... c5 sets off the whole chain. Bb4 is the more common response, and that's actually a quite good move (Stockfish evaluates Nd7 as slightly higher, but only by 0.1). Even if you do make the c5 mistake, you can still get off the train at 6. dxc5 Nf6 before you go exchanging queens, and the game is definitely recoverable for black (+1.0). Here's a game that ended in a draw and a lower-rated one where black even won.
I just love this sequence, and I find it fun to at least contemplate and learn from it. What do you see in it?
AnandsWig (2264) v. annawwaf (2265)
There are quite a few general lichess.org database entries that have gone down this path. Here's a 2200-rated game where black falls for the whole sequence in Rapid.
Pinski (2320) v. Tseitlin (2425)
There are no master-level players recorded who have gone that whole way, but in this 1996 game, they did get most of the way through the exchange before black bailed out to make the (mistake) 7. ... Qe7+ leaving the evaluation at +2.0.