Rounded corners seem so simple it’s easy to think that you couldn't mess them up. Unfortunately there are a couple ways people often go wrong.
1. Squishing Corners
Most modern tools will take care of this for you, but always check to make sure your corners are properly proportioned. Any time you convert a shape to simple paths or rasters you risk distorting your corners when you resize.
2. Offset Radius Centres
Rounded corners with outlines should have a consistent weight along the entire path. Nested objects should have a similar relationship to keep the gap between child and parent consistent.