@Stuart ********
Depends. In my experience, flying from Europe (Switzerland for instance) they don't really care but flying from Vietnam, they now do. Australia and the USA are strict as well. Australia will let you through if you hold an onward ticket to another country (even Cambodia) but they might question you for a bit before giving you a boarding pass.
Laos and Myanmar don't check; Singapore generally doesn't either.
None of this is a hard and fast rule, but it's based on my own experiences.