Someone posted an interesting picture of forked citrus leaves in an external forum (here). Has anyone encountered this before? What causes it?

I'm not sure of the cause, but would guess it is genetically caused. There could also be an association with calcium deficiency. I'm surprised to see that it came from the UBC Botanical Garden forum. That forum died years ago, because that forum's administrator, became too busy to do his original job. - Millet

Citrus questions can and continue to be posted to the UBC forums. It's just that they no longer have a dedicated forum. Of course they don't receive as much attention since Citrus Growers members who used to frequent that place are no longer there.

One of the symptoms of a calcium deficiency in citrus are leaves that develop a heart shape indentation at the tip of the leaf. - Millet