Eraser doesn't respect the locked alpha toggle. For example when using white pixels in an alpha locked multiply layer as a mask this regularly causes me to have to go back and repair it since I only notice it much later since the change is invisible until I try to paint on the layer again. Reproducible: Always Steps to Reproduce: 1. Make a document with only 1 transparent layer 2. Paint on the layer with a brush 3. Lock alpha of the layer 4. Hit E and erase Actual Results: Alpha is modified despite the alpha lock. Painting on the layer becomes impossible until alpha is unlocked and re-created. Expected Results: Alpha remains the same. Maybe paint with background color instead (like photoshop), do nothing. Subsequent paint strokes are visible and constrained in the expected area.
Alpha lock meant to make sure you don't draw outside the lines, not to prevent you from erasing what's there, so this works as expected, I think.