Rob saw this at a golf course snack shop. Maybe the "tip" is acknowledging that while it makes sense to tip a barrista who makes your complicated coffee drink, tipping for a coke out of the cooler is really charity.
I don't usually post the following: newspaper headlines, personal email, craigslist postings, unprofessional websites. I also tend to not crosspost things from other blogs, since I have so much unique material waiting for me to get to it.
Things I see a lot: silica gel "do not eat"; hair dryer labels; inside the bus "do not drill"; Wal-mart sign about IDs; coffee machine with "2" cup sizes; employees must "wash hands".