QGIS » QGIS Application

QGIS allows user to copy\\paste a style where the origin layer style has different geometry type than the destination.

Although this might sound like a good thing, it breaks the layer rendering in the canvas.

From that point on, the layer won't render in the canvas, and you can't correct the problem via layer properties since it "blocked" to other type symbols. The only way to reverse this is to copy a style from a layer of the same type, or reload the layer.

To replicate:
1 - Add two layers of different geometry types (e.g. Lines and polygons);
2 - Copy the style of one of the layers and paste to the other;

Probably paste option should be disable if the destination layer is not of the same geometry type as the origin.