Sometimes the narrower 6-pin RJ11 plug will damage the outer pins in the 8-pin Ethernet socket. Those outermost pins are pressed back by the edges of the RJ11 plug.

This internal spring/pin damage usually doesn't matter unless you later try to use that same jack for real Ethernet purposes.

My own approach is to not trust 8-pin jacks for Ethernet if that jack has ever had a 6-pin 'telephone' plug inserted. If I want it to be reliable for networking I will usually remove and discard that jack and install a new one.