I was teaching a class related to consuming data from a web based service on an iOS application. In the worksheet I mindlessly used a placeholder domain of x x x . com, with the intention of replacing it with the actual domain name once I deployed the server code to test against. I forgot to update the placeholder before I printed off the worksheets and in the middle of the class I started hearing some chuckles.
I was confused until someone spoke up. "What is x x x . com?"
Everyone had started typing in the domain name from my worksheet into their browsers.
Me: "Oh. No."