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

intenational.jpg
Spelling error: Intenational should be International - missing an 'r' (I didn't actually discover this one. I found it online)
boysbathroom.jpg
Apostrophe error: should be BOYS without an apostrophe (from a HIGH SCHOOL bathroom...sigh.)
2011-07-07_14-11-18_99.jpg
Usage/homophone error: are should be our (From a restaurant in Panama City Beach, FL)
2011-06-06_19-24-17_403.jpg
Punctuation error: Sentence should end with a question mark (From a restaurant/store in Pigeon Forge, TN)
2011-06-06_19-22-52_289.jpg
Apostrophe error: Change to children's (From the same store as the previous image)
2011-06-06_19-19-56_847.jpg
Apostrophe error: TEE'S should be TEEs (apostrophes show possession; they do not make a word plural)
2011-05-24_08-17-24_928.jpg
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
2011-04-07_20-06-34_153.jpg
Apostrophe error: No apostrophe needed in SUNDAYS (The word is plural, not possessive) - at a local gas station.
2011-06-10_18-24-11_817.jpg
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.