There might be instances when you're not at fault for missing your flight, in which case, you're much more likely to receive some kind of compensation from the airline or have a stronger case for a refund.
Missing a connecting flight
One of the most common reasons for missing a flight is if you have a connecting flight and a delay with the first flight causes you to miss your second flight. In this case, your flight is protected by the airline and they'll put you on the next flight and supply you with any necessary accommodation or catering free of charge.
Please note: Connecting flights are only protected if the same airline is providing the connection or if the two airlines providing connections have an interline agreement (a pre-agreement to handle the same passengers and their baggage on both flights). If you book a flight on Alternative Airlines where the connection is from two different airlines, you can be sure that these airlines have an interline agreement. The only time you might have booked a connecting flight without an interline agreement is if you book two separate flights independently.
Flat tire rule
This isn't a rule that is publicly documented but the 'flat tire rule' is an unofficial rule that an airline might apply on a case-by-case basis. The flat tire rule means that airlines will assist in putting you on the next available flight for free or a small fee, as long as you can provide proof that the reason for your delay was beyond your control and you arrive at the airport within two hours of your flight leaving.
The rule name 'flat tire' comes from the idea that flat tire is a valid reason for missing a flight beyond your control. If, for example, you can provide a receipt of your breakdown cover, the airline would most likely apply the flat tire rule. As the flat tire rule is unofficial, the leniency will differ from airline to airline. But, it's more common with US carriers, such as Delta, Southwest, American Airlines and United.
If you've checked-in
If you have checked in for your flight, the airline might be a bit more lenient in helping you out. This is especially true if you've checked in on time and were delayed due to airport issues, such as long security lines.