For the American cousins:

The phrase, "Greengrocer's apostrophe", derives from the hastily chalked boards or hand-written cards on tables of produce inside shops or on the pavement [sidewalk] outside, or at market stalls, advertising such things as, "Potatoe's - 15p / lb" and "Carrot's - 30p / lb"

You could club them over the head with Chamber's "Modern English Usage", and they'd still do it.

...

The "double negative". The best take on this I ever saw was in Alan Bleasdale's TV series, "Boys from the blackstuff". Yosser Hughes (played by Bernard Hill) is facing a tribunal from the Department of Employment.

Hughes: I didn't do no work for him.
Adjudicator: That's a double negative.
Hughes: So ? There's two of you, aren't there ?

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The Lands End / Land's End debate; how about the other end of Britain. Is it "John o' Groats" or "John o' Groat's" ? I suspect the latter, but the two road atlases I possess each give a different version.