An unexpected error has occurred. Your purchase has not been completed.

Double check your payment method and that you have enough funds for it. I get that when trying to buy stuff and don't have enough in my card.

When the new community votes come up steam gets messed up for a while. Combined with the load from all the people downloading L4D2 that might be all it is. Try again in an hour or so?

No help for Cheaters, sorry.

Been there, done that.
If you see that error, here are notes for the future.
First, if you get the error 3 times in a row, stop and wait 10-20 minutes.
If you get the error again 20 minutes later wait 1 hour.
If you still get it, try again in another hour.
If you still get it, your account may have been messed up trying to purchase during the server overloads. Last time that happened to me, took about 3-4 days to clear. Thankfully it was outside one of the sales.
Just tips I've picked up over the past few months reading the Steam Help & Tips discussion forum and seeing this come up quite often. Also, from having been stuck once on the long side of waiting (thankfully not during a sale).

I had that error, I just restarted my computer and internet connection and deleted the cookies and htmlcache folders in steam/config or whatever directory it is.

I had that error if you are buying from other country you need to use the link http://store.steampowered.com/?cc=XX where XX is your account country US, IT, DE, etc.

had that error when using steam family sharing (beta), but now everything ok :|