Such a tiny punctuation mark. Hardly anything at all but potentially misleading if omitted or misused!
The omission I saw was on one of those electronic boards over the motorway - M1 on Saturday - and made me chuckle. I was in pedantic mode as I often am when reading. Here's what I saw:
The 'bin it' was on a lower line because the upper line had been all used up.
I reckon that a comma was needed after 'litter'.
Strictly speaking (or reading), the above says 'don't litter bin it' i.e. don't put your litter in a bin. If indulging in even greater grammatical pedantry, I would say a hyphen between 'litter' and 'bin' was omitted.
Still, back to the comma or lack of it. A comma would render the above a suggestion. Had a full stop, or even an exclamation mark, been used after 'litter' it would have become a command. No: a comma would be preferable in my opinion.
don't litter, bin it
As for getting commas in the wrong place, how many of us do that? Jane Austen seemed to use a comma where one would stop to take a breath if reading out loud. Other writers seem to have a bag of commas into which they thrust their hand after writing a page of words, sprinkle the page liberally - or with a veriest pinch - and go on with writing.
For the most part, errant commas may result in mild irritation but beyond that, what do they matter?
Well, this example shows the comma could make the difference between life and death:
Kill not, to fear the deed is good.
Kill, not to fear the deed is good.
Personally I'd go for a colon after 'kill not' but colons are another matter altogether :)