Steam Purchase Failure: An Unexpected Error has occurred, your purchase was not complete.

Contact Support?

Try to add funds to your wallet instead of purchasing the game directly

Try transferring the money from the mastercard to a paypal account and using the paypal account.

Use Paypal! :)

Try web browser store instead of steam?

Could be that your region's fucked up in your cookies. Try clearing cache and cookies in your browser, log in to steam again, and try purchasing through the browser instead of the client.

If this works, reinstall Steam by deleting the whole Steam folder, except for steam.exe, and your steamapps folder. This will purge all user-specific stuff, including cookies. Run steam.exe again and it'll re-download Steam for you. And no, you won't have to re-download your games - that's why you keep the steamapps folder. You'll just have to run through your Steam settings again quickly.

Contact Support. It can be a region problem or they blocked your account from buying things.

And of course it can take up to weeks until one response :D

If the card was stored in the system already, try telling it to use a different card and then input it again. Sounds strange, but actually has worked for me with quite a few places.

Or like others said, try adding to your wallet first or Paypal.

it depends, I'm not sure but I submitted a ticket and it got answered in about 24 hours :P

Ive been getting this error when I tried to use my paypal in the US:/ I dont know why but I really want to buy terraria and this is preventing me from doing that

i got this error last week but the sale actually went through, i never realised until i looked at my uninstalled games list.

This happened when I changed address recently. I updated my address with everything but my bank. Steam and GreenManGaming kept declining payments - not sure why some companies choose to check the address and others don't. Guess some are considered at higher risk of fraud than others.