Something I discovered.

Last week, I started TF2 as usual, and Steam said me that the inventory files or something like that does not match with server so it asked me to either upload current files to the server or download from the server. I chose to download and BOOM. The weapon I crafted was back. So, if we can modify the client-side inventory and show it as older, can we trick TF2 to create non-existent items?

This may not work, please don't say things like "OMGZ ITZ NUT POZIBL LOLOL N00B". I know it is impossible to hack items. This is just an idea for skilled programmers.

This some times happens when they do updates, actions in the game get rolled back by a few minutes or seconds.

I was just browsing some threads and found this one.
Yes it is possible (ONLY WHEN THIS HAPPENS) to do it.
The only problem is... It's kinda rare to encounter, so it isnt fully possible.

Um.... Well uh um uh. I don't know.............

I used to encounter this when I started playing TF2, for the first 3 weeks. After that, it no longer happens where does the inventory file get stored anyway?

Everything is posible nothing is impossible.