**System Information**
Archlinux, Intel i5 (Sandy Bridge)
**Blender Version**
Broken: 2.72b
**Short description of error**
Snap to pixel rounds wrong for negative coordinates and is not idempotent.
**Exact steps for others to reproduce the error**
Open {F131328}.
In the UV/Image editor window press Shift+S and select "Selected to Pixels".
Repeat the last step and notice how vertices are shifted although they already lie on the pixel grid.