Reproducible: Always When drawing on paint layers while rapidly and repeatedly using the "Activate next/previous layer" shortcuts to switch between them, one of the layers will suddenly behave as if it is in a locked group. Steps to Reproduce: 1. Create a new document with 2 layers 2. Alternately press page-up and page-down while drawing circles with the mouse. Press and release the mouse button while drawing so that you end up drawing on both layers. The on-canvas popup message "Group not editable" will appear when the bug occurs.
I've seen this happen as well.
Resolved with the fixing of bug 376214