Default ambient lighting in Unity introduces “negative light” effects when the skybox has a huge dynamic range. See the following screenshot. This scene has a very high-contrast red skybox and a single white directional light. This skybox introduces greenish shadow on the lower half of the sphere.

https://imgur.com/SD29Xz9

These artifacts get worse in 5.3. See the following screenshot taken in 5.3.0f4. The lower half of the sphere is lit with clearly wrong color.

https://imgur.com/2Z3lo6x

This issue makes significant differences in rendered results when using HDRI cubemaps.

Note that this issue doesn’t reproduce when ColorSpace == Gamma.