Excuses for selector slipups:
1) Cockpit gremlins changed it whilst you weren't looking
2) Hostie knocked the selector with her knee
3) The vibration from No2 shook it
4) The Copilot has been fiddling again
5) The chick from row 3a knocked it as she was leaning over to look out of the window
more ???
not that I make this mistake on a regular basis !!!!