Odd thing this afternoon and something that I had never considered.

Measured some jointing on site and wrote the measurements on a sketch.
Walked 50yds to workshop and laid out the joints on 6x3" beech. Subject for
the timber is immovable.
Just before starting to cut the joints I thought measure twice / cut once.
So took the laid out timber for a walk to its new home and compared.
Blow me down. The markings for the joints were all to cock.
Baffled.

Turns out that the two tape measures I was using (on site & in workshop)
differed by approx 11mm over 1M.
I have checked and verified this. Both measures are same model Stanley and
similarly aged.

Cleaned the timber, re-measured all using one tape only, laid out again.
Compared on site again. Cut joints. Fitted. Job done.
Damned hard world if you can't trust a tape measure or two. This one won't
catch me out again.
A little lesson learned perhaps.
Nick.