You bet they do. But like all things the more money you have the better you get treated.

Small retail investor, your margin call is likely automatic with assets sold without your input to take the money.

large hedge fund. Tables of lawyers deciding how much you have to put up, at what time, and what can you move out of that bank before you pay.

Never read the book (http://www.knightmareonwallstreet.com/) but watched it happen in real time from the trade desk at a different firm.

You could also say it happens to small banks. If you fail some sort of FDIC testing, you're classified as at-risk and they force you to sell to a larger bank so as not to risk depositor funds.