I make hollandaise (egg yolk, lemon juice, butter) in the blender using Julia's quick method from her first book. The trick is to pour in the oil (or melted butter, in my case) slooooooooowly.... more a stream of droplets than a pour. If I do that, it always works.

FWIW I never manage to load it up with more than 1/2 cup or so per yolk that way, but it's still a recklessly indulgent topping for asparagus on a random weekday night, and only takes a couple minutes to assemble. :-)