People writing tonne when they mean ton, as in it cost a tonne of money. It's a wonder they don't write it cost a tonne (0.984 Imperial long tons) of money.
One of my colleagues does this all the time. If I'm editing his copy, I usually change it to 'loads' or something else to avoid the issue.
I think it's a form of hypercorrection, whereby all units are pedantically expressed in metric even if used figuratively.
See also: hide your light under 36.387 litres, go the whole 8.23 metres, give them 2.54cm and they'll take 1,6km...