2 throwable items exist in TF2 that go unused. These were presumably intended for weapons at some point before the weapons themselves were cut. These projectiles are all that remain.
When this throwable hits a player, it does some damage and pushes them back. It attempts to use the model
models/weapons/c_models/c_balloon_default.mdl, which does not exist in the game.
A basic throwable which uses the bread model as a placeholder. It deals damage when it hits a player.