It's an occasional pest on lemons occurring more on the coast but seldom in the Central Valley.
The bud mites enter the buds and suck out the sap, the result is a misshapen ovary hence the fingerlike projections.
I've had a few lemons every year infected with these bud mites and I never thought anything about it thinking it was the tree's natural mutations. The 95% of the lemons looked like lemon they tasted like lemons why worry about some stray weirdos? Besides they're kinda cute. Then I ran into the UC IPM article which reads: “Even as high of 80% of buds infested with this mite may not cause consistent or predicable damage to the developing fruit.” Which looks like the case in my tree. Before budding, I'll apply some horticultural oil and see if it does the trick.
The misshapen fruit still has all the other characteristics of a lemon. It tastes like a lemon-smells like one too. When citrus bud mites give you misshapen lemons make lemonade.
For further reading: The Citrus Bud Mite, Eriophyes sheldoni Ewing and An illustrated guide to Plant Abnormalities Caused by Eriophyid Mites in North America (PDF).