Slaying of the Witch king

[Essex]

'Look!' he cried; and stooping he lifted from the ground a black cloak that had lain there hidden by the darkness. A foot above the lower hem there was a slash. 'This was the stroke of Frodo's sword,' he said. 'The only hurt that it did to his enemy, I fear; for it is unharmed, but all blades perish that pierce that dreadful King. More deadly to him was the name of Elbereth.'

Yep, it only got his cloak, and yes, maybe if it HAD pierced his skin it could have had the same effect as Merry's sword.

But IT DID NOT HAPPEN. So glorfindel's propehsy still stands. It was like second site. It did not happen BECAUSE of the propehsy, if it did, the prophesy would have been different.