Unix Millennium Bug
The Unix Millennium Bug started getting noticed (in 2006), exactly a billion seconds before its sceduled execution, after there were some mysterious crashes which were finally traced to the definition of time_t in unix-like operating systems (and more generally, in the ubiquitous C (the programming language)). From a thread in PIClist, I also found this webpage which claims to have been written in 2003 (in case the opening paragraph looks familiar to you, yes, the whole page is a mostly plagarised version of the programming challenge in the introduction to the excellent book Expert C Programming by Peter Van Der Linden, a book I would recommend to anyone).