Usually scored dies are from dirt/debris on the brass being inserted into the die. You could have picked up some dirt from anywhere.
I have done this after being extremely meticulous with my steps also. I will say that not cleaning the dies was most likely my culprit also.
What I chalk it up to is that there was something still on the case after being tumbled, didn't notice the crap, and scratched my die. What I DO know is once this is done, the die will not stop etching your brass, unless you would repolish it. Better to just buy a new one.
And I will back up 98's question, I am assuming you just missed typing in a step for us to read.