Matt explains, "The Julian Doves and Desperados are a reenactment group that performs 'historic, comedy, gunfight skits' outdoors on Sundays." I wonder how many people spit in this spittoon?
Submitter Tiffany says that this is on the door of the bathrooms at the Langley Porter Psychiatric Institute at the University of California, San Francisco. Perhaps "knock first" is their slogan.
Marianne attributes this one to "a friend's dad, who was either very proud of his newly installed sink or deeply skeptical of his DIY plumbing skills." At least it's not "water"...