Could the stock options or RSUs you receive from a “unicorn” startup be diluted?

Question (Orig. on Quora):

Of course the rsu’s can be worth nothing if the company implodes, but other than the company simply failing, is it possible for my stock to become worthless?

Answer (by Dan Walter):

Sure. Imagine the company goes through a down round financing. Imagine the new investors put a structure in place that ensures they receive their full value if the company is sold below a given price. If the company is eventually sold below this price then all of the value of the company would be paid to the latest investors and nothing to the holders of employee equity.

This is just a super simple hypothetical but I can bet that some people on Quora have real horror stories about similar transactions.