In an article about the life of Grothendieck, available here:
Allyn Jackson writes about how Mumford was profoundly impressed:
Mumford found the leaps into abstraction to be breathtaking. Once he asked Grothendieck how to prove a certain lemma and got in reply a highly abstract argument. Mumford did not at first believe that such an abstract argument could prove so concrete a lemma. “Then I went away and thought about it for a couple of days, and I realized it was exactly right,” Mumford recalled.
What were the lemma and proof that so impressed Mumford?
(I have tried asking algebraic geometers and category theorists; the tags attached to this question are speculative.)