9999 bug
n. A potential computer software problem where a program might confuse the date September 9, 1999 — 9/9/99 — with "9999," the end-of-file code in some older programming languages.
More than 30 years ago, many programmers inserted "9999" to indicate the end of a file or data transfer. Others, faced with a date entry and unable to leave spaces blank, put down Sept. 9, 1999 — 9/9/99.

The worry is that computers still working with old COBOL or FORTRAN software will misread all those nines as permission to stop everything in its tracks.

Most technology experts agree that the 9999 bug will not bite.
—Murray Campbell, “Today's 9999 glitch a drill for Y2K bug,” The Globe and Mail, September 09, 1999
The main problem with the 9999 bug was it had been eclipsed by Y2K and taken companies by surprise, according to PricewaterhouseCoopers' local IT and systems integration leader, Mr Fred Balboni.

"It's an interesting issue because it has not got the publicity or board-level attention that Y2K has. It also hasn't got the budget and is something many people have just woken up to during the last few months," he said.

Compared to Y2K, though, the 9999 bug would be a "non-event" as it only affected old mainframe systems, Mr Balboni said.
—Mandy Bryan, “Screening at local terminal 9999,” Australian Financial Review, September 06, 1999
1998 (earliest)
Conformity with rule 3 will also ensure that Delta will not be impacted by the 9999 bug — a problem caused by early programmers indicating the end of a project by entering the date 09109/99 or 9999.
—“DGD,” AAP Newsfeed, June 26, 1998