You're running your program, and then it suddenly exits with the message This application has requested the Runtime to terminate it in an unusual way. What happened?

That message is printed by the C runtime function abort, the same function that also causes your program to terminate with exit code 3.

Your program might call abort explicitly, or it might end up being called implicitly by the runtime library itself.

The C++ standard spells out the conditions under which terminate is called, and it's quite a long list, so I won't bother repeating them here. Consult your favorite copy of the C++ standard for details. (The most common reason is throwing an unhandled exception.)