If you messed up your homemade mayonnaise, it's more than likely from getting in a hurry and adding too much oil at once. Make sure, no matter how close the mayonnaise is to being finished, you do not add too much oil. It will immediately thin it out. To thicken it back up, pour the thin mayo into a measuring cup. Place an egg yolk in the empty food processor. Pulse the yolk a few times. Start the process over of slowly pouring the oil and egg mixture back in with the egg yolk. Do this very slowly while blending. If it turns out thin again, you are adding too much oil at once. This process can be repeated until you achieve a thick mayonnaise.