ON

The first time I lived properly independently was in my second year of university, with two friends.  When one of them tried to use the (electric) shower, she came out in a state of distress in her dressing gown proclaiming "the shower's shouting No at me!"  It transpired that she had read "ON" upside-down.  Obviously we will never let her forget it.

I was (fondly) reminded of this when I stuck this label on a torch, and could anticipate similar issues.