Without using recursion how can a stack overflow exception be thrown?

Since no one else has mentioned it:

throw new System.StackOverflowException();

You might do this when testing or doing fault-injection.

Declare an ENORMOUS array as a local variable.

If you call enough methods, a stack overflow can occur anytime. Although, if you get stack overflow errors without using recursion, you may want to rethink how you're doing things. It's just so easy with recursion because in an infinite loop, you call a ton of methods.