Quality and reliability

Main articles: Software quality, Software testing, and Software reliability
Software quality is very important, especially for commercial and system software like Microsoft Office, Microsoft Windows, Linux, etc. If software is faulty (buggy), it can delete a person's work, crash the computer and do other unexpected things. Faults and errors are called "bugs". Many bugs are discovered and eliminated (debugged) through software testing. However, software testing rarely – if ever – eliminates every bug; some programmers say that "every program has at least one more bug" (Lubarsky's Law). All major software companies, such as Microsoft, Novell and Sun Microsystems, have their own software testing departments with the specific goal of just testing. Software can be tested through unit testing, regression testing and other methods, which are done manually, or most commonly, automatically, since the amount of code to be tested can be quite large. For instance, NASA has extremely rigorous software testing procedures for its Space Shuttle and other programs because faulty software can crash the whole program and make the vehicle not functional, at great expense.