I’m one of those pedantic people who point out mistakes in other people’s written literacy: including text messages; greetings cards and signs.
Grammatically incorrect signs annoy me more than anything else because their purpose is for people to see and look at them, so in short, to read them. But, if it is grammatically incorrect, it could have a 
completely different meaning.
EG: A Panda eats shoots and leaves.
A Panda eats, shoots, and leaves.
This is the obvious example that is used so frequently among linguists. But this example comes from this joke:
A panda walks into a café. He orders a sandwich, eats it, then draws a gun and proceeds to fire it at the other patrons.
‘Why?’ asks the confused, surviving waiter amidst the carnage, as the panda makes towards the exit. The panda produces a badly punctuated wildlife manual and tosses it over his shoulder.
‘Well, I’m a panda,’ he says, at the door. ‘Look it up.’
The waiter turns to the relevant entry in the manual and, sure enough, finds an explanation. ‘Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots, and leaves.’
Sort it out.
