A friend of mine was trying to buy the gem hat. He bought it once, it took his Units, but it never gave him the hat. He bought it at second time, and it took his Units again, but didn’t give him the hat. He didn’t receive the items until after he closed and re-opened the game, where he now had two of them in his inventory.
I tried to buy a hat at around the same time and I just got “Transaction failed.”, so I’m assuming it was likely a delay on Steam Inventory’s part?