Final answer:
The 250 knots speed restriction below 10,000 feet may not apply in mountainous regions, to military aircraft under certain conditions, or if waived by ATC due to traffic or safety reasons.
Step-by-step explanation:
Speed restrictions of 250 knots below 10,000 feet do not apply when the aircraft is operating in airspace where the minimum safe altitude is higher due to terrain, for example, in mountainous regions.
Additionally, the speed restriction may not apply to military aircraft during certain operations, or if an air traffic control (ATC) waives the restriction for a specific reason, such as traffic flow management or if it's necessary for the aircraft to operate at a higher speed for safety reasons, like avoiding severe weather.