Unless you have the patience of a saint, you probably don't mask off all the glass when painting windows and doors. If you do, there is nothing more frustrating than discovering paint has somehow worked its way around the tape in spite of your best efforts. While there may not be a foolproof method of keeping paint off glass, there is a way to remove the paint once it's on there. This paint removal method works on all kinds of paint, including frosted paint and old paint that has been on the glass for a long time.
Fill a bucket with 1.5 gallons of warm water and add about 2 tablespoons of dish soap. The dish soap is meant to act as a lubricant, so mix it into the water a bit but don't stir it long enough or violently enough to make a lot of soap bubbles.
Wet a sponge or rag with the soapy water and run it over the painted areas of the glass. Saturate the rag enough to make the glass wet but not so soggy that it drips water on your fresh paint.
Score a line along the edge of the window pane with a utility knife to separate it from the surrounding paint. Only go deep enough to slice through the layer of paint between the glass and the wood.
Scrape the paint off the window gently with a razor blade or paint scraper. Hold the scraper at a 45-degree angle when scraping so you don't scratch the glass. It does not matter which direction you scrape. Periodically wipe the paint off the scraper blade as you work.
Dry the window with a soft cloth, then wipe it down with glass cleaner for a streak-free finish.