Cycles spotlight creates fireflies on glass and glossy materials Its about spot light lamps in combination with Cycles Glossy and Glas Shaders. In this post i only make use of unchanged shaders "Glossy BSDF" and "Glass BSDF" from cycles. 

The blend file is here: http://pasteall.org/blend/index.php?id=47499

And what i am trying to do is a simple Cycles setup using a camera, a flat cube and a spot light: 




The Spotlight has Strength 300 with a spot shape size of 20 degree
The cube material uses Glass BSDF (Beckmann) with roughness 0.1 and IOR 1.45
The camera looks straight downwards
All items are aligned on the Z axis


This is the render result with 128 samples after about 15 seconds:



Now i want to get rid of the fireflies. I followed the recommendation to use more samples. I increased the number of samples from 128 to 1024 which created this image:


The downside here is that now the image renders in 2 minutes and actually the spot is not so much "spotty" but more "cloudy"

In the next experiment i increased the spot Strength from 300 to 1000 and now the flies are back:


Someone told me that increasing the lamp size will fix it. So i increased the lamp size from 0.2 to 1.0 and the flies are gone but the spot is again cloudy.



Ok, lets make a break here. Actually i am not even interested in the reflection on the glas surface (for now). I rather want to get the spot light passing the glass and travel further down to another flat cube that uses a glossy material (a mirror actually).

I change the setup a bit as follows:



So i now expect to see the reflection of the light spot on the lower plane, and indeed the spot is there, on the left side i see the reflection from the glas and on the right side i see the reflection from the lower plane (the mirror:



but ... fireflies are back :( Now i need some help out of this :) thanks for any hint!
Try using the denoise feature in 2.79.

Using the default denoise settings with your file at 1024 samples (and 100% resolution):



A different option without denoising is to modify the default glass shader (see :How to illuminate the darkness inside glass objects in Cycles?).

(1024 samples at 100% resolution)