When a friend wants you to go out for a beer but you're skint so he offers to buy you a beer, but you still say no. The next week your out together and you say he has to buy you a beer because it's a beer roll-over (like the Lottery roll-over), in fact because it's a roll-over, he owes you two beers! Lotto Beer!
Friend "Hi mate coming out for a beer?"
You "Nah, I'd love to but I'm skint."
Friend "I'll buy you a beer!"
You "No it's alright, I'm staying in".
A week later in the pub together...
You "Are you gonna get me a beer?"
Friend "What? Get your own!"
You "You owe me one! You said last week you'd buy me a beer!"
Friend "What??? That was last week!!!"
You "Yeah it's a beer roll-over!"
You "You owe me a beer, so it's a beer roll-over!"
You "In fact you owe me two beers cos it's a Lotto Beer roll-over! Get 'em in!"