Here is a short demonstration on a phenomena called the colour banding problem. The following cameras are affected by this: NX20, NX200, NX210, NX1000, NX1100. The problem leads to coloured circles when lifting the shadows.
Look at this example:
The image above is taken with the NX20 and shows the colour banding problem. Below you can see how the image should look like, it is taken with the NX30.
Both shots ware taken in RAW with EVC set to -3. I postprocessed the images with Lightroom 5 and I lifted the shadows by +100 and set the EV to +1. Probably this problem is a result of a badly implemented vignetting or distortion correction. Therefore it doesn’t occur on images taken with non native lenses since they are uncorrected.
What do we learn of this? Be careful when underexposing your images with the affected cameras. Try to expose for the foreground without blowing the highlights. Choose a good balance between shadows and highlights. This is generally a better idea since lifting the shadows will result in a lot of noise, too. Another option is to use High-dynamic-range (HDR) imaging techniques.