I remember when I was just out of college, I was warned that there was such a thing as a mortgage that did not allow for extra payments, and that you should check for that when applying.
If you couldn't, or even if you did the payment incorrectly, anything extra would just be treated as if you sent your payment in for the subsequent month a little early. I've heard of the latter happening to friends, but I've never seen or heard of the former.
Many loans you have to be careful that you don't pay them like that. If you don't specify that you're paying down principal with the extra, it just goes toward next month's payment. So next month you might only owe $50 instead of $500.
If you couldn't, or even if you did the payment incorrectly, anything extra would just be treated as if you sent your payment in for the subsequent month a little early. I've heard of the latter happening to friends, but I've never seen or heard of the former.