There are instances where a company/government undergoes debt restructuring, in which, assuming bondholders agree, the issuer can extend, consolidate, or refinance its debt at a lower interest rate, longer duration, etc. Bondholders could agree to swap their current bonds for bonds that mature at a later date, so at the maturity date of their original bonds, they won't receive the par value; instead, they'll receive the par value of the new bonds at the new maturity date.
This isn't necessarily equivalent to a default, because the issuer still agrees to pay the par value of a bond at its maturity date, but it is a situation in which you might not receive the par value at the original maturity date. This is often different from a default, because bondholders may still receive the par value of a bond at its maturity date, even though it's not the par value of the original bond at the original maturity date.
However, a debt restructuring that extends the duration of your bond may open you up to an increased risk of default. Since there is more time left before the new bond matures, there is more time for the issuer to find itself in financial distress and default on its debt. There isn't any guarantee that it won't decide to either restructure its debt again, undergo a different kind of restructuring, e.g. a debt-for-equity swap, in the case of a company, or default before the new bonds reach maturity.
There is another situation where you may receive an amount greater than or equal than the par value, but not at the maturity date: callable bonds. With these bonds, the issuer of the bond is allowed to buy the bond back from the owner at a specified call price.
Since the call price usually exceeds the par value, this isn't necessarily a concern, but it's something to keep in mind if you're basing an asset allocation or investment on the assumption that you'll hold the bond to maturity and receive the par value at that time.