Massively

Linden Lab advises that 521 simulators have been failing to save state since 3AM SLT (US Pacific) this-morning (that's every simulator that was updated to version 1.19.1.80264). That means no changes to land or objects in those simulators is being remembered. To resynchronize them, they'll be restarted and rolled back to the previous software version - which will means that they will reboot to the last saved state (essentially forgetting everything) [edit: including land alterations, object placement/removal and land sales!!]

Linden Lab urges you to grab any objects that you have placed in those simulators since 3AM this-morning, and to stop making any adjustments to those regions until the restarts can take place (commencing at 7PM SLT). Check the full list of the affected regions to see if one of yours is involved.

UPDATE:

It appears that the Linden Lab devs managed to figure out a way to get the ailing simulators to do proper state-saves, so in the end no rollback of code or data was necessary. It took hours longer than the original plan, but was definitely a superior result for residents.

A handful of simulators restarted themselves due to crashes and other glitches, and those had to be dealt with, but a tip of the hat to the Linden Lab devs for finding a better solution to the problem than the original plan.