Here's one that's often considered hilarious:

A man walked into a bar and ordered a beer. As he sipped his drink, he heard a voice say, Nice tie! He looked around, but there was nobody nearby who could have said it.

A few minutes later, he heard, Beautiful shirt! Again, he looked around, but he couldn't find anyone who might have spoken.

A few more minutes passed, and he heard, Great haircut! This time, he decided to investigate. He asked the bartender, Did you hear those voices?

The bartender replied, Oh, that's just the peanuts. They're complimentary.