mystral: Not sure about Singapore, but in France it's mostly the decision of your coordinator and the relevant person who runs the course.
Some professors are more lenient than others on prerequisites, others stick to the rules.
Usually if you have a good explanation for wanting to bend the rules or they like you, people will let you get away with it.
I managed to have some of my CS courses recognized as some of the "language" credits needed for my History master (avoiding having to retake courses on Latin), so some coordinators are
very lenient.
Yeah, there's a rule that if you have under 14/20 for your master, you can't enter in a doctorate course, but that rule never really applies. I got a 14/20 and some years later I could enter the doctorate course without any problem.
mystral> yay ! a fellow french man who graduated in History like me ! ^_^