Mechanical failures are always possible, but these days they are rare. Modern systems can be built to tolerate shocks and extremes of temperature. And as an added layer of security, critical systems like those on the space shuttle will often run three or more sets of hardware in parallel. Even then, it is rare that a manual check will explore every conceivable action a program may perform. Modem software's main problem is complexity.
Computer programs are often elaborate million-line mathematical statements.
Even the best programmers will make a handful of errors in every thousand lines of code they write. When programs don't work because of an inadvertent error, the problem can be almost impossible to find. Things are now getting even more complicated because a great deal of modern software runs on networked computers. Spin has generated particular interest among space-exploration researchers. It's tricky to put things right when the software is running on a computer millions of kilometers out in space.