This is the photo that was submitted to go with my SF Chronicle (sfgate.com) Golden Gate Gardener column today. How could they resist printing this wonderfully sculpted Eureka lemon? But resist they did, so I thought you'd like to see it here. The image was taken by Dagmar Zidek, whose lemon tree has the problem.
This happens because some mites enter the flower buds and start sucking out the sap. The ovary of the flower is misshapen, so the fruit is, well, outlandish. Citrus bud mite is apparently particularly a problem near the coast in our area, just where we depend on lemons for most of our garden citrus.
The goal, in managing the pest, is to kill it without killing too many of its predators, in particular predatory mites. You have to spray when the bud mites are active and not yet inside the buds, which is May and June, then again in September through November. Use a summer oil spray. Some summer oil formulations are still based on petroleum, but apparently studies showed that vegetable oils worked fine too, so now a lot of the newer ones are based on vegetable oils, like canola or soy. So keep your lemons looking like lemons by keeping this pest under control!
Or maybe you'd rather go for the Deformed Lemon Hall of Fame.