On 26 January at 12:49 a.m. MST, the 2nd Space Operations Squadron at the 50th Space Wing, Schriever Air Force Base, Colo., verified users were experiencing GPS timing issues. Furtheri nvestigation revealed an issue in the Global Positioning System ground software which only affected the time on legacy L-band signals. This change occurred when the oldest vehicle, SVN 23, was removed from the constellation. While the core navigation systems were working normally, the coordinated universal time timing signal was off by 13 microseconds which exceeded the design specifications. The issue was
resolved at 6:10 a.m. MST, however global users may have experienced GPS timing issues for several hours.

Source: [time-nuts] Fwd: CGSIC: FW: Official Press Release – GPS Ground System Anomaly

Timing issues of 13 microseconds can make huge huge differences in accuracy. Very scary, especially considering the BAe / Russian / European systems are not fully implemented as a backup for most people.