The atomic clocks in the GPS satellites were set with base dates in January 1980 and the indicator for the week was set at a maximum of 1024. This means that on August 22, 1999 the atomic clocks will reset themselves to week 0. GPS receivers, navigation equipment and time-keeping equipment which has not been properly programmed to deal with this change will experience problems when the week counter is reset -- but it is not clear how serious these problems will be.
Incidence:
The bug is expected to be more of a problem with older equipment (pre-1994) than more recent equipment.