The U.S. government bonds are usually thought of as risk-free investment. However, it might be a risky investment in the long term. Let’s say you have money that you don’t need for the next 20 years, and the annual inflation rate is 3% ~ 5% during the period. Historically, the returns on the Treasury Bills were a bit more than those of the inflation rate, and in many occasions, it failed to exceed the inflation rate. However, the returns on stocks have always been superior than the inflation rate in the long term.
The idea is simple. If your return is below the inflation rate, your purchasing power declines. You can’t buy the same amount of goods as before. Since the “risk free” investment (i.e. Treasury Bills) are more prone to the danger of giving returns under the inflation rate than “risky” stocks, I would call “risk free” as risky in the long term.