If we want Murphy to screw with Germans for real, how about sudden weather change on September 1st around noon? Let's say it starts raining - and keeps raining for a week. Luftwaffe is grounded. German armoured advance is much slower, since many Polish roads change into mud - it might not be so bad for tanks, but tracks with supplies get stuck. Poles have time to mobilize rest of their forces using railways, not so affected by weather.
To be honest I'm not so sure if it would have saved Poland, but eventual German success would have been much more expensive.
To be honest I'm not so sure if it would have saved Poland, but eventual German success would have been much more expensive.