What I meant, and I failed to explain this properly, was that the amount of equity you need to have in the account increases as the stock or index rises, and ev

What I meant, and I failed to explain this properly, was that the amount of equity you need to have in the account increases as the stock or index rises, and eventually it can happen that the position requires more money than you have, and you're hit with a margin call. You're correct that if you already have enough money in the account to hold the position, you can continue to short sell.