I'm still trying to figure out what "leftover cake" is. I've heard of sponge cake, chocolate cake, fruit cake, banana cake, coffee cake... but leftover cake? What do you put in that?

Or is it this mythical thing like "leftover wine" and "leftover chocolate" that recipes seem to think exists but I've never been able to find in the shops? I hate this "leftover" brand because they all use it and I can't find it!