Sketchup Round Corner Crack Top ((link)) -

A common mistake is using too many segments (e.g., 12 or 24) for a small corner. Lowering this to 6 segments is often enough for a smooth look while keeping the geometry manageable and less prone to breaking.

SketchUp has difficulty forming very small faces (under 1mm or 1/16 inch). When you round a corner with a high segment count, the resulting faces may be too small for SketchUp to "heal," leaving a gap or crack. sketchup round corner crack top

If you prefer not to use extensions, you can use the tool: Curved face problem - SketchUp Forums A common mistake is using too many segments (e