Pesky mayonnaise! Most of the time, making it with a stick blender works perfectly and quickly. (see this post)
It makes nothing but a runny, disgusting, oily mess. Sometimes it can be attributed to user error (most commonly, being impatient about letting the egg come to room temperature) but sometimes it seems to be a reason unfathomable, like the phase of the moon or which side of your head you parted your hair.
It’s sad to throw out the ingredients and start over. A sad, sad waste. It’s enough to make a person completely lose control, throw up her hands and buy a bottle of mayonnaise.
Before you do something so drastic, the Damsel has recently come upon a fix. When a mayonnaise emulsion doesn’t work, they call it “broken,” and so it is. But if you’ll invest just one more little egg yolk, all will be well again.
Put a room temperature egg yolk into a container. Add the “broken” mayo to it drip by drip while whisking madly. (you can use your stick blender if you wish) You can increase the drip rate to a small drizzle after half of the liquid has been incorporated.
Magic has returned to the earth!