Offer your thoughts or opinion on the input financial query or topic using your financial background.

Is there any circumstance in which it is necessary to mark extra payments on a loan as going to “principal and not interest”?
It could be a couple of things besides extra principal: I seem to remember hearing that some (shady?) lenders would just pocket extra payments if you didn't specify where they were headed, but I've also been told that this just isn't true.