Yep, happens a lot on my lessons! Usually after ive been taking them through backing-up to a TFTP server! You really have to stress the "N" that goes after the erase flash before copying prompt!

It is highly unlikley that it is a fault in the router if it only happens every now and then - but i suppose it is possible.

AND - when I finished the practical part of a lesson I have an ACL I enable that will deny all traffic exept from my machine to the routers - you always get some smart-ass who thinks its funny to erase the flash memory or change the paswords etc which usually results in me missing my most of my lunch!