1. Create a color layer, draw a few lines
2. Create a second color layer, artifacts appear along edges of drawn lines.
The artifacts are present without adding anything to the second color layer - even if you turn off the visibility of the second layer. There are other layer types that can be chosen for the second layer that will also cause this to happen, but color seems to be the most striking. I've noticed there seems to be a particular problem when overlapping black, as well.
Here are a couple examples from my last drawing. This drawing has 1 normal layer, 2 color, 1 multiply, 1 color burn, 1 difference, and 1 overlay. I couldn't save as an image file (and not get artifacts) without first merging down the first color layer.
Unmerged:
Merged: