I've been told the same thing by two different blues in the past two days. Personally I find it nonsense, and can't find any reference to it in the rules (ISF) but am happy to admit I'm wrong.
I stand with a wide stance, with my left foot on the furthest edge of the left side of the pitching rubber. If it's a raised rubber, my left foot will have the toe in contact with the plate, and the heel of my right foot with also be in contact. If it's a flat rubber, my left toe will be over the rubber, and the very back of my heel will be on the right side.
Both umpires told me this weekend, that my left foot must be inside the peg. (the peg that holds the rubber down) I clarified with both umpires that my foot was touching the plate and they both confirmed it was.
Is my stance illegal?
(to make it clear as mud, I'm putting up a picture showing my stance. Marvel at my awesome Paint skills! Black is obviously the rubber, red is my feet, and the yellow is where the pegs would be.