What was the most recent situation in which you couldn’t get your point across to one or more other people?

I was "discussing" the pros of getting vaxxed with one of my nephews and one of my nieces, both of whom are anti-vaxxers.  No point I made seemed to change their minds.  At this point, I've stopped trying.

This is minor, but my sister was discussing her drug usage in her younger years and said she used OTC diet pills for energy. I could not convince her that it was amphetamines.

An argument over censorship with a mod on this very site. Even JustAsking conceded it was "pretty tame", but wouldn't overrule her.