The Y2K Bug Simulator adds filters to most core WordPress functions that report back a date, rendering 19xx in place of 20xx.
The Y2K Bug Simulator partially mimics one of the potential y2k failures that could (and did) affect some systems after December 31, 1999. The specific failure it mimics is one which typically occurred in systems where years were represented using two digits rather than four. Thus, in an affected system, the day after December 31, 1999 might be rendered as January 1, 1900 (in this scenario, only the last two digits of the year roll over, the first two are hard-coded as "19").
The bug that this plugin mimics was certainly not the only y2k bug, nor, arguably, a terribly significant one. It is, however, perhaps the most aesthetically interesting. The effect of this plugin on WordPress is to change 20xx dates into 19xx dates across page titles, posts, and comments, as well as archives and calendars.
Some things to note: