Grrr

Just about suitable for Tiny Tuesday (thanks Dbifulco for hosting).
This cost me a lot of time and effort this morning. You're looking at the inside of my old and usually trusty tyre pressure gauge. I'd just attended to six of the seven tyres on my car and caravan when I realised that the reading  for the seventh couldn't possibly be right. My second gauge, new digital, and awkward to use, revealed that all the pressures were hopelessly incorrect.
Gritted my teeth and started all over again.
Now I'm puzzled, these things are so simple, how can they possibly go wrong?