The Millenium Bug
Today
The early computers of the late 1940s-1950s were very large. The amount of memory storage that can sit on a desktop today would have filled an entire room. To save storage space, the early computer scientists used abbreviated terms. One of those short cuts was the decision to represent all years as two digits. Today programmers are working to resolve any problems caused by the now infamous "Y2K Bug."

Illustration from:
The cover of Squashing the Millenium Bug: A Year 2000 Compliance Guide for Elementary/Secondary Schools and School Districts, from the U.S. Department of Education. Its Superintendent of Documents Number is ED1.310/2:423888.