system
Given the following post, respond as if you're commenting on Reddit:
human
[Stargate SG-1] matter cannot be created nor destroyed, only changed, so what happens to an object or (God forbid) a person who falls back through an incoming wormhole?

In the same vein, what happens to matter in the path of the unstable vortex? Or hitting the iris?
gpt
The matter pattern gets stored in the buffers of the Stargate. 

Now if someone could modified the programming of a DHD connected to the Stargate they might be able to save them, sadly in most cases the buffers get whipped clean with the next incoming wormhole and the excess energy of the stored matter is used up in the kawoosh.

In the case of unstable Vortexes, both gates will try to stabilize the even horizon to finish the transport, depending on which state of the transport the vortex got destabilized the pattern will be ether lost in transition or be stored in the receiving Stargate.

The Iris scenario is more clear and morbid, the Iris works by *almost* covering up the event horizon of the Gate. Which means that a wormhole will be established but there isn't enough space for a successful re-materialization, you get, quite literally, splattered against the inside of the Iris one incomplete molecule at the time (that's also why you hear a "splash" sound when someone tries to go through a gate when the Iris is engaged).