if a 2-conductor TS plug is inserted into a 3-conductor TRS socket, then the socket's ring is shorted to ground, thus any signal sent from that socket's ring is lost. Equipment not designed for this short might, for instance, damage an audio amplifier channel.
if a 3-conductor TRS plug is connected to a 2-conductor TS socket, normally the result is to leave the ring of the plug unconnected.
3.5 mm TRRS sockets that accept TRRS headsets (stereo headphones with a mic) are often compatible with standard TRS stereo headphones, whereby the contact that expects a mic signal will instead simply become shorted to ground and thus will provide a zero signal
TRRS headsets can plug into TRS sockets, in which case its speakers may still work even though its mic won't work (the mic's signal contact will be disconnected)
due to a lack of standardization in the past regarding the length given to the ring conductor and the insulating portions on either side of it in 6.35mm phone connectors and the width of the conductors in different brands and generations of sockets, there are occasional issues with compatibility between differing brands of plug and socket. This can result in a contact in the socket bridging (shorting) the ring and sleeve contacts on a phone connector
two incompatible video variants exist, of 15 millimetres (0.59 in) and 17 mm (0.67 in) length, and using the wrong variant may either simply not work, or could cause physical damage.
attempting to fully insert the longer (17 mm) plug into a receptacle designed for the shorter (15 mm) plug may damage the receptacle, and may damage any electronics located immediately behind the receptacle. However, partially inserting the plug will work as the tip/ring/ring distances are the same for both variants.
a shorter plug in a socket designed for the longer connector may not be retained firmly and may result in wrong signal routing or a short circuit inside the equipment (e.g. the plug tip may cause the contacts inside the receptacle – tip/ring 1, etc. – to short together).
the shorter 15 mm TRRS variant is more common and physically compatible with standard 3.5 mm TRS and TS connectors.