Lubos Fric discovered a line for white that absolutely crushes black for playing c5. I fell victim to it when I played him a centaur match, and he also obtained a completely won position on ICCF when he played it there, though he defaulted on time for some unknown reason. As such, the line has gone virtually overlooked in the databases. Nevertheless, it's
extremely potent:
1. d4 Nf6 2. c4 e6 3. Nf3 b6 4. g3 Bb7 5. Bg2 c5 6. d5 exd5:
7. cxd5! Bxd5 8. Nc3 Bc6 9. e4!
This double pawn sac puts black in a very bad situation. Play continues:
9...Nxe4 10. Nxe4 Bxe4 11. Qe2
And white gains a powerful initiative worth well more than the two pawns. If you plug the position into an engine like Houdini, the eval will steadily get worse and worse for black with each deeper ply. In my centaur game, I saw this and tried 8...Bxf3, but still got slowly outplayed by Fric's computer until I was a pawn down in a lost endgame.
Here's his ICCF game where he defaulted in a completely winning position:
And here's where my alternate attempt to defend the black side failed in our centaur game: