9/9/99
The second pre-Y2K milestone occurred in the following month. In many programs or data sets, "9/9/99" was used as a rogue value to indicate either an unresolved date or as a terminator to indicate no further data was in the set. This raised issues when the actual date this represents, 9 September 1999, actually arrived.
Read more about this topic: Time Formatting And Storage Bugs