As much as I understand, that "forced monetization" can exist, such as in the case of stock options - those can incur tax right away (alternative minimum tax), and usually will have to sell some of your just got stocks to be able to pay for that (if you don't have other assets to cover it).

For example, point 2 at http://www.forbes.com/2010/03/10/10-tax-tips-stock-options-p...