Thursday we had a surprise roll out to the RC channels of the main grid. The package was a fix for a security exploit. Friday early in the morning the update rolled to the main channel of the grid.
So, what was going on? The details were published by April Linden 2016-02-19 11:18 AM in the SL Forum/Blog. See: Why the Friday Grid Roll?
This exploit was in part of Linux system software. The Linux developers made the patch. The Lindens had to figure out what it would do to the Second Life(TM) system. Was it needed? Yes, crackers could exploit SL. Would the patch break anything in the SL system? Nobody knew. They had to find out. So, the Lindens put a lot of effort into getting this exploit resolved.