Steam loco damage at slow speeds?

Hi all,
I've been asked an interesting thing today and have been racking my brains to find some answers?? Does anyone know or can you think of a fault for a steam loco where it can still continue a run at a lower speed to conserve more damage?
Thanks in advance
Andy

I seem to remember one of the preserved Britannia's stopping somewhere and taking one set of valve gear down before proceeding at a slower speed but I can't remember what the cause was.

Pete