Windows 10, Nvidia GTX 980
Short description of error
Not sure this is truly a bug, but: It's possible to make a negative lamp in Cycles by giving a light a negative value for strength. However, when using light sampling threshold, the negative power will fall below the threshold and sampling will constantly get canceled for the negative light
Exact steps for others to reproduce the error
Add a lamp with a negative strength, note the difference in effect between light sampling threshold of 0 (disabled), vs any larger value (enabled).
A test blend with a negative lamp is located here: