I have been struggling with this issue for some time now. Every once in a while, when I render a frame it will have bizarre...glitches, is all I can call them.
Animations are where this is most noticeable and prevalent, by nature of them requiring so many frames. However, this issue has occurred on single frames as well.
Two main types appear. Most frequently they are large glowing areas, usually a highly saturated red (but sometimes other colors as well). They vary in size greatly. The other is a full-on static effect that covers a significant portion of the image. (Both can be seen in the attached video). It seems almost random, as a re-render seldom produces the same results.
I believe this might be caused by the Glare compositing node interacting with fireflies and the denoiser, but I am not entirely sure. It is almost certainly not a compression artifact, as it is visible in the Image Editor even before being exported as an image or video (I think it happens in the compositing stage). I am not sure if this is a glitch, or what, or if my diagnosis is even correct; but it has been plaguing me across countless .blend files for some time and I have been unable to figure it out.
I am using Blender 2.80 Mac. CPU rendering.
(The attached video has a very low resolution and sample count, but the same problems occur with better qualities and lighting as well.) https://youtu.be/tJpcI5edvic