Trying to change version string in mIRC.

Indeed. Anything which raises an exception *could* be used as a breakpoint. INT3 and DBx registers are only one way to skin the cat. They raise explicit debug exceptions, but a debugger that can't catch other kinds of errors would be a pretty lousy debugger. For that matter, don't just limit your thinking to exceptions; various system APIs can also be used to invoke the debugger. It's easy to think of "breakpoints" with an overly narrow mindset. Get creative