BNCconnectors

[ppppenguin]

Some manufacturers originally used a thinner centre pin to get the 75 ohm impedance match.  The matching 75R sockets can definitely be damaged by 50 ohm plugs.  I have done it!
There was also one type that had a very needle-like pin not a blunt pin for the 75R version.  But all this became immaterial  when the manufacturers realised that all they had to do was remove the ptfe insulating collar round the pin in the 50R plug section and bingo, 75R with the same pin diameter, using air dielectric.
So the only mix that causes problems is 50R plugs into any original thin-pin sockets.
Simples...
Jeremy

Presumably this means that 75R sockets designed for the thin pin plugs will also get stretched by 75R plugs that have the thicker pin.

At the sub GHz frequencies where you're likely to use BNC connectors the matching error caused by the different impedances hardly matters. The professional digital video world uses BNC up to 3GHz which is a bit marginal for many types of 75R connectors.