A month after it was installed, a combination of circumstances made a software error manifest and every long distance switch shut down causing a nine hour nationwide communications blackout. Scientific American Magazine compares the current state of software development to the state of industrial development before Eli Whitney invented the assembly line in 1799. In other words, software is essentially a handicraft item. A single wrong letter in a statement of code, in a critical spot, can crash a program. Also, two identical machines can have chips with identical properties and specifications but were programmed by different programmers in different countries so that one machine is year 2000 compliant and the next one isn't.
Programmers write a program to specifications.
Hopefully the person that wrote the specifications understood the job he was describing, and hopefully the programmer understood the specifications. The program could just quit, or go into an infinite loop, or follow some other unplanned for chain of logic. But it will probably not do what it is supposed to do. This may have few consequences, or many. A badly programmed chip could turn a switch or Programmable Logic Controller off line, or it could have a cascading effect like 1990 and knock the country off line.