mickey
Don't know for sure what caused the missfire but I am thinking that something maybe polluted the powder before the bullet was seated.
Yes the primer had fired and that's what lodged the bullet into the lands.
I dug the powder out of the missfired case and it came out in clumps and was a yellowish colour instead of the usual grey.
When I got home I pulled all my other reloads from that batch and could not find anything wrong with them.
Perhaps I managed to get a drop of bore solvent into a couple of cases or something similar.
I'm pretty sure it must have been my cockup anyway and it had the immediate effect of tidying up my reloading practices!

Several years later I bought 200 new 458 cases some of which had been reloaded with a dupont powder of unknown type and Hornady 350 grainers.
I pulled these and was shocked to find that the powder had to be dug out of the cases with a screwdriver and some had powder granules stuck to the base of the bullet along with signs of some form of corrosion.
Some bullets had powder stuck to them and some just had corrosion while others looked quite normal.
I think I still have some of these bullets so maybe NitroX might be kind enough to post a photo if I do one and email it.
My 500 grain loads are well compressed but have never given any problems.