Sometimes abbreviated as Y2k, the Year 2000 bug was a warning first published by Bob Bemer in 1971 that described the potential issues of IBM computers using a two-digit year date stamp. In the past, to save time and space programmers represented a year as two-digits instead of four, e.g. '79' for '1979'. If a computer using this code rolled over to the year 2000, the computer would interpret '2000' as '00', causing the computer to think it is '1900' instead of the year '2000'.
Because many computers at the time were still using this code including many banks, governments, hospitals, utility companies, etc. many thought this could cause mass hysteria and potentially result in the end of the world. After millions of dollars were spent upgrading computers to accept the year 2000, no significant issue or end of the world events occurred on January 1, 2000. Because no significant problem occurred on new years eve, many hailed the updates successful, while others considered the bug as a hoax or overblown.
Also see: Bug