I've had some troubles with the Cycles Denoising (which is a wonderful feature).
In some projects it produces anomalous black bars and/or black spots on my renders.
I did some testing with the newest nightly build. The artifacts are only present when using GPU for rendering, CPU produces flawless images:
You can also see that the GPU render produced a skewed noise pattern. like there is some motion blur in the image, although nothing is animated ,and it's the same scene, so it should look the same. I used very extreme settings to trigger the problems, but they sometimes happen in real-life scenarios without resorting to stupidly low sample counts. I initially thought that the issues are triggered by a holdout shader being used in the scene, but it doesn't seem to have any effect.
Here's my test project: