Cambodia, like some other countries, requires travelers to perform a COVID-19 test within x hours. Example. What happens if a flight to Cambodia is late by 1 or several hours and that as a result the COVID-19 test a travel performed is not anymore within x hours before the flight/boarding?
The rules for various countries are subtly different. Some require a test within x hours of departure, others within x hours of (scheduled) arrival, with or without provisions for unforeseen events. If the rules do not allow for exceptions, carriers would presumably have to deny boarding.
As an example, the Netherlands requires a PCR test within 72 hours of the scheduled time of arrival with an extra 24 hours for involuntary delays. This is as specific as it gets, boarding time is not relevant but there is a provision for short delays. This also means that a longer delay would be treated as having no valid test.
Germany's rules are even stricter. Depending on where you have been in the last ten days, you are required to present a test performed at most 48 hours before entry, no grace period or exceptions to my knowledge. If you consider the length of the flight and the time it takes for a lab to return a result, this leaves a very short window of time to get tested but that's still the rule. If that's not possible where you are, you might end up being stranded with no quick way to return to the country.
Another question is what happens if you make it to the border without the required test (maybe airline staff failed to notice the test wasn't up to the required standard, there was delay after departure, or you somehow managed to cheat your way onto a plane). What I have witnessed is that you would have to take a test then and there and be “invited” to isolate yourself (or join a managed isolation programme of some sort). As another example, France asks carriers to check you have a form consenting to all this in advance, should the need arise.
In case of USA, it seems that you would need to be re-tested if your flight is late and has fallen outside the window of X hours. This is what I found on the U.S. Embassy website in The Czech Republic.
Q: What happens if my flight is delayed on my way to the United States and the 3 day time frame expires? Will I need to retest?
A: If your flight is delayed before departure, you will need to get re-tested if the delay causes your test to fall outside of the 3-day pre-departure testing period requirement. A delay while traveling on a continuous itinerary will not invalidate an otherwise valid test unless it results in you leaving the airport terminal or a layover lasting longer than 24 hours.