This has happened twice to me today when returning home after travelling elsewhere on the Grid. The ambient light from the HDR EEP I've set up here breaks, resulting in areas without local lights going dark. The lighting returns to normal after I relog.

Screenshots attached to this report, and here's SLURL of my home parcel if anyone's interested in trying to reproduce it: http://maps.secondlife.com/secondlife/Sawdel/173/9/107

I've not been able to trigger this bug in Megapahit or any other viewer so far.