Three people have dinner in a restaurant. The meal costs $30, they each give $10 to the waiter. Waiter goes to the cash register and realises he has overcharged them, the meal only cost $25. Waiter thinks "These bastards didn't tip me, so I'll keep $2 and give them back $1 each." So, the waiter keeps $2 and gives the diners back $1 each. So each of the diners has paid $9. And the waiter kept $2. That's $27 plus $2, making $29. Where is that other dollar?

This all made sense last night when I was drunk.

That's $27 plus $2, making $29

Why did you PLUS $27 with $2? You supposed to MINUS $27 with $2. That's $25 you've paid for dinner!

Like I said, it made sense when I was drunk, and I'm hungover now. :)