A friend bought a new tuner and came down to our regular session proudly displaying it.

Played first song and I went to join in (allowed!) and realised that I was way out of tune. Quickly retuned to him and carried on with him for the rest of the song.

A little later I went to join in with the fiddle players and got very black looks as I joined in a semitone out. Quick retune etc etc

It then turned out that John's tuner had a 'useful' semitone up/semitone down button which he must have pressed by mistake...

Though it is useful sometimes if you are playing with box players and don't want their help.