It's one of those rivalries that knows almost no bounds :-D
I was once (about 10 years ago now) visiting a place in Derbyshire called Matlock with some friends. They have this quaint little steam train service to Buxton, but it wasn't running that day.
Anyhow, we were just looking around the station there. One of the shelters looked burned out at first, but when we got closer, we saw it was covered with marker pen graffiti from Nottingham / Derby fans. We couldn't believe it, staring incredulously at the density of it all around the walls!
Every square inch was plastered, except for a clear space on the far wall.
On closer inspection, we saw it was a single piece of graffiti that both sides had completely respected and left a six inch space all around.
It read:
"Belper is a slum"
For some reason we p*ssed ourselves laughing at this! We drove through Belper on the way back. It's true!