I've frequently made homemade mayonnaise in my food processor following Alton Brown's recipe, and consistently had good results.
Last week the food processor was busy with something else, and so I decided to make the mayo in the stand mixer. Using the whisk attachment and turning it up to high speed, I got everything emulsified but the resulting mayonnaise was very runny -- not broken or separated, but much less viscous than what I get from the food processor. It's been in the fridge for about a week, and is still the same consistency (emulsified but runny).
I assume it got too much air whipped in or something. Is there a way to use a stand mixer for mayonnaise, or does it really need the speed of a food processor or blender?