Taking advantage of a known problem with the Operating System or any
running services on the target, a good programmer can build an
application that sends some data that causes the targeted system to
crash.
The worst case scenario is not when a hacker crashes a service, but when
it finds a way to maximize the CPU usage causing a total malfunction on
the system.
Of course most of the attacks are not initiated by these kind of
programmers, but by "script kiddies" who only read about it, find the
program's source over the web and just use it without knowing the
mechanisms it uses.