I was looking at the description of espace étalé (see Wikipedia’s article on sheaves) in J.S. Milne’s lecture notes on etale cohomology where I saw this scary sentence: “It is possible to avoid using these spaces — in fact Grothendieck has banished them from mathematics — but they are quite useful, for example, for defining the inverse image of a sheaf”. Well, I hope for Milne’s sake that Grothendieck never finds out.