Y2K
Sometimes abbreviated as Y2K, the millennium bug or Year 2000 bug is a warning published by Bob Bemer in 1971 describing the issues of 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 rolled over to the year 2000, the computer interpreted '2000' as '00', causing the computer to think it's '1900' instead of the year '2000'.
Because computers were still using code with this problem, many thought this could cause mass hysteria and potentially result in the end of the world on January 1, 2000. After millions of dollars had been 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 Year's Eve, many hailed the updates as successful, while others considered the bug as a hoax or overblown.
Y2K was featured as a top term of 1999.