http://en.wikipedia.org/wiki/IBM_AP-101
http://en.wikipedia.org/wiki/Space_Shuttle
PS: It's easy to forget just how old and hacked together the Space Shuttle is: Historically, the Shuttle was not launched if its flight would run from December to January (a year-end rollover or YERO). Its flight software, designed in the 1970s, was not designed for this, and would require the orbiter's computers be reset through a change of year, which could cause a glitch while in orbit. In 2007, NASA engineers devised a solution so Shuttle flights could cross the year-end boundary.[38]
It is. The 5th has just enough to get the shuttle home, and is designed by a separate team. The presumption being that anything that knocked out the first 4 is probably a systematic failure.
A GPS string might erroneously say it's day 367 or be one character too long because it printed 20010 or a telemetry link might ask for 4million readings because it calculated "-1"