To clarify, let's work through an example. Consider an unary encoding: 0 is 0, 1 is 10, 2 is 110, 3 is 1110, etc. In this case, if we feed the decoder an infinite sequence of 1's, it will remain forever undecided as to the integer's value. The result says we can find such pathological inputs for any other encoding system, not just unary.

I'm not sure this is particularly significant. If you haven't finished feeding in the input you can hardly expect the machine to give a complete output unless you have given a specific specification as to how to simplify or truncate. I perhaps would avoid saying that the decoder 'hung'. 'Hung' implies some sort of pathological error in the decoder, not just a decoder that is processing new input as fast as is possible in exactly the way it is supposed to.

That said, many things that seem bloody obvious and trivial are considered to be important mathematical principles once phrased in maths language. Maybe what you are describing is significant. It does seem like the sort of thing that usually gets a mathsy name. "Infinite Garbage In, Garbage Out" may not cut it. :)