What happens if you accidentally put windshield washer fluid where the antifreeze goes?
If windshield washer fluid was accidentally put in where antifreeze should be, expect rusting and heat ups. Also expect engine to overheat quickly.
What should you do if you accidentally added radiator fluid to the windshield washer fluid reservoir?
You do not want your windshield washer fluid to become solid in temperatures below freezing, because this could damage your washers and prevent them from working. You should not, however, use engine coolant/antifreeze in your washer fluid, because it is toxic and can damage your paint. Commercial washer fluids contain a weaker antifreeze that is much less toxic and unlikely to damage your paint.