Strange light on surface without an origin

[Kadri]

I had a strange light on a surface of a mountain..
Disabled all lights, searched for luminosity on all surfaces of objects etc.
Couldn't find the cause....
Just when it begun to be annoying i remembered the GI cache.
There where some light changes (i think) that i forgot and didn't care.

Took me nearly 1 hour to find with all the render tests. 
So when making changes, making a new GI cache render is the way to go in most cases probably.
This is an animation so i hesitated to do it just because of render time.
Lost maybe more by not doing it in the first place...Doh!

[WAS]

Oh yeah, this has happened to me a few times. Wondering why lighting and shadows is all wonky, then remember I changed the sun position.

[Dune]

I had some 'aha' moments with that too, a few times.

[Kadri]

It looked to me  that there is another problem too with "Light source" point lights in animation (or GI ).

It kinda reminded me on this problem because the wrong light pattern kinda changes with every render (not sure) :
https://planetside.co.uk/forums/index.php/topic,29008.msg285967.html#msg285967

Without "GI cache" it looks Ok. But with it, even with 1 frames blended or 2,3,4,5 etc. it is not what you expect.
With very high strength at least. I had to use "1 000 000" to see it. But it is there with smaller numbers too probably.

I tried it without any displacement on the terrain too. So not related to reflection or such.

The first image below is without cache. The second one with cache.
The GIF example is with much lower strength. You can see the strange light on the right side

This is with v4.1.25
I wanted to test the new version too but the free version doesn't have cache support.