Holy cow, I wrote a book!
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.
That message is printed
by the C runtime function
the same function that also causes your program to
terminate with exit code 3.
Your program might call
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
(The most common reason is throwing an unhandled exception.)