I guess it depends...It is possible to put something on your hair that's too greasy, or makes it too stiff, or builds up too much. But that's more a result of using the wrong ingredients in a product.

Also, for hair as curly as ours, I leave in a good conditioner to weigh my hair down. For someone with fine, limp hair that's easily flattened, this would totally flatten their hair. So in that case, their hair would be over-conditioned by leaving in conditioner like I do. But for very curly hair like mine, I want to weigh my hair down, so leaving in the conditioner makes my hair moisturized, clumped, and happy.

If you leave in conditioners that have damaging ingredients in your hair (like salt, and some others), then that won't be good for your hair and may cause breakage. And then the issue wouldn't be over conditioning, but leaving something on your hair that's got harmful ingredients in it.

But having constant conditioning (from products that have no harmful ingredients to your hair) is good. There is nothing in the conditioners I recommend that will hurt your hair. If this were damaging, I would not have been able to grow my fragile hair down to my hips. But this won't work with any conditioner. Some will hurt your hair if you leave them in. That's why I specify conditioners that I've found to work really well, on the site.