McDonald's restaurant to grab a quick snack. At the counter there was a sign advertising two apple pies for 99 cents. It sounded like a good deal until I looked up at the big menu board and noticed that the price for one apple pie was only 49 cents. I wish I had taken a picture of the two signs. When I shared this story on Twitter, Danny Nicholson replied with a link to his Bad Maths Flickr group.
The Bad Maths Flickr group contains lots of examples of bad math stopped in stores and other public places.
Applications for Education
The Bad Maths Flickr group could be a good place to find some images that contain simple mathematics problems for your students to solve. For example, ask students to find the flaw in the math of this grocery store offer or this offer for cat food.