Richard,

A radio amateur, by an easy mistake, uses a 75-ohm plug and socket in a
50-ohm coaxial transmission system. The total length of the plug plus
socket is 1"

As a result of the mismatch what is the SWR produced on the 50-ohm line at 2
MHz. At 30 MHz? At 150 MHz?

Is the amateur, or anyone else, likely to be aware of any difference in
performance?

No need to make measurements. If you are unable to make a simple calculation
and answer the question then you are not qualified to continue the
discussion.
----
Reg, G4FGQ