Some real world errors I've discovered and used as grammar warm-ups in my classes:

Spelling error: Intenational should be International - missing an 'r' (I didn't actually discover this one. I found it online)
Apostrophe error: should be BOYS without an apostrophe (from a HIGH SCHOOL bathroom...sigh.)
Usage/homophone error: are should be our (From a restaurant in Panama City Beach, FL)
Punctuation error: Sentence should end with a question mark (From a restaurant/store in Pigeon Forge, TN)
Apostrophe error: Change to children's (From the same store as the previous image)
Apostrophe error: TEE'S should be TEEs (apostrophes show possession; they do not make a word plural)
Usage error: Everyday should be two separate words (You could also argue that there should be a colon after Watermelons) - from a local grocery store
Apostrophe error: No apostrophe needed in SUNDAYS (The word is plural, not possessive) - at a local gas station.
Usage error: it's should be its (its = possessive; it's = contraction of 'it is'), and the sentence needs a period. Also, this is just an awkward sentence. It needs revision. This is from a music software box.