Items with a durability get reset to 100% when being distributed. So it's possible to use a repair pack until it's at 1/300, and then ctrl+drag it to a chest and find it back at 300/300.

The same goes for ammo, and even science packs.

And damaged items (a damaged turret for example) will also get fully repaired when distributed.

This bug existed for a long time but is really hard to fix. I don't think it's worth the hassle since in Factorio you generally have tons of items and having one more or one less doesn't really make a difference. Just don't abuse it ;)