Why was a drop immediately discarded?

I failed played some more TF2 today, and reached Medic milestone 3. I thus got a ubersaw. However, it was automatically discarded for some reason.

What happened? Is my backpack full? Is this item just locked for me?

Solution 1:

If you received an item that didn't appear in your backpack until later this is not unheard of. Usually it's a glitch or a synching error, but the item will usually be held for you and added to your backpack when it can. Restarting TF2 sometimes will help with this.

Solution 2:

This appears to simply be a relic of days long gone. Indeed, clicking on 'view next' gives me:

Note how this item was "found", while the "DISCARDED" item was an [UNKNOWN] item acquired through a medic milestone (at least, that's what I imagine the monocolor medic image is about.)

Solution 3:

On the screen that notifies you of the drop, there's a delete button. Is it possible you hit that without realizing it? It would probably be difficult, since I believe it also displays a confirmation to prevent such accidents.

Have you checked out your backpack? It's possible your backpack is full. If you don't want to go all the way in game, you can check here.