Not as bad as the Holocaust, but it comes very close.
It deleted my HL1 save files, and I had to completely uninstall Steam/re-install HL1 and delete my old saves (I tried copying them) for it to even WORK again. I tried running it outside of Steam, and it was Steam that messed it up.
I don't like Steam because while it is "revolutionary" in the sense it allows you to buy games online and keep them forever if you have the password to your account, it is slow as hell and unfair to those on dial up. I had dial up when I bought HL2, only to spend 3 hours online updating/unlocking the files to find out I CAN'T RUN THE GAME.
And it was not me who messed up the HL files. I didn't install any mods, didn't mess with the files, I just booted it up one day and got an error, and when I restarted the game got the same error.
Steam sucks.
I had a blog. It sucked.