Speaking of...

It's fun to purposefully mislead people; luckily, conversation is a great vessel for this.

As well as xkcd's awesome example, I like to say "speaking of [something]" as a precursor to a topic, when said topic has never even so much as been alluded to. It's best to do with confrontational people—you then get to try and convince them that you had definitely mentioned it and they probably weren't listening.

Sometimes boredom is a dangerous thing.