Yes, I've heard of this A LOT.
So many people come and say to me "I wish I could have bf'ed, but my milk was too thin." Or, "He got too greedy."
I think several factors play into it:
1) Doctors look at formula fed babies as a guide to growth standards (at least they do over here in the U.S.) and breastfed babies rate of growth is different from the way that formula babies growth pattern. So therefore a mom with a normal healthy breastfed baby who is not as fat as a formula baby at that same age would be thinks that maybe her milk is not enough, and that her baby needs to gain more weight.
2) Misunderstanding of growth spurts. Mom's don't understand how their bodies work, and think that because baby hit's a stage when the are marathon nursing, that they have suddenly stopped making enough, and feel the need to add supplements.--Not understanding that this is your baby's way of telling your body to make more.
3) Expressed breastmilk looks different from formula and cows milk. It looks thinner and they think that since their milk looks thinner, and their baby is not growing as fast as other (formula fed) babies, and since their babies are suddenly nursing, like ALL the time, then something must be going wrong, so they introduce that first bottle of formula.