Hark! "Credulous" hath been inscribed upon thine rafters!

If you have a sharpie and a bartender that isn't paying attention, writing "gullible" on the ceiling of a bar once can net a lot of free drinks over time.

Especially in a place with lots of tourists and not many regulars.

But if you were gullible, you wouldn't bother to check. You'd just take the word of the person at face value.