Update: When we said we didn't think Valve would take too long to fix this? Our hunch was correct, and users should be able to redeem their Steam gifts and inventory items now without a hitch. A few minutes ago, Valve replied, "This issue should be resolved, sorry for the inconvenience."
Looks there there's an issue on Steam currently that's keeping some users from redeeming their gifts and inventory items. Early reports have popped up on the official forums, and Valve has already responded with an assurance that they are aware of the problem and developers are trying to resolve it.
Multiple threads popped up reporting the same issue, though the original in question reads:
just received a gift and already redeemed?
i just received a gift (tc2) and its already been redeemed? it doesnt make sense, my friend just bought it and gift it to me just now. this must be a glitch please fix for the product i've just purchased from my friend!
Before long, numerous other users popped up to announce they encountered the same problem. We don't expect Valve to take too long on this, so just sit tight a moment.
The official Steam Support Twitter also made a post announcing that the developers know about the gift/inventory redemption issue.