Overspray On Windows

A trick that you can use to help ensure that you don't dig into the glass with a razor blade is to wipe one edge of it on your jeans for a handful of passes. Basically like you're strumming a guitar with a pick. It'll curve the blade ever so slightly. Just make sure you use it with the curved part away from the glass.

We used to do this when detailing vehicles to get old dried on parking passes off the back of plastic rearview mirrors. It worked without digging into the plastic.