You should always check the small print. Even if it's invisible to the naked eye.
Here's a poster on a BART platform that Jon C came across.
The usual 'terms and conditions apply' disclaimer is at the bottom right. However, it's so small we've had to enlarge it for you.
But, if you think that's small, check out the asterisk next to it, and then try and find the text to which the asterisk refers.
Here's a clue. Even at the full poster size you'd need a magnifier.