Hidden danger - Sofa Spud
Hidden danger of courteous act!

If you signal that you're letting someone pull out of a junction, they might take it as a sign the road is clear and get hit by another vehicle.

Or if someone in the right turn lane flashes you out of a junction, that doesn't mean the car passing them on the inside will stop! Crunch!

It's easy to wave or flash someone into a near miss or worse, even when you're aware of the risks.

Happy motoring, Sofa Spud
Hidden danger - none
One dark, wet and windy night I was driving along a narrow town centre road.
A car coming uphill towards me seemed to have his headlamps on full beam so I gave him a quick flash of my full beam, that didn't work so I slowed down and gave him another quick flash. That worked - he pulled out in front of me to complete his right hand turn. Performed my first emergency stop for many years and caught sight of his face looking towards me as he came broadside on - sheer terror all over it. He must have though I was some sort of psychopath, as I though he was, until realization dawned.
Hidden danger - Sofa Spud
Headlamp flashing can be confusing. If someone appears to be flashing their lights to let you go, they could be flashing at the vehicle behind you because they know them.

Cheers, SS
Hidden danger - frostbite
Or be a Volvo with running lights going over a bump. Still catches me out once a year.
Hidden danger - hillman
My workmate told me that his wife was very indignant with a motorcyclist who had collided with her car. It transpired that another lady had signalled her that she would give way and let my workmate's wife out of a side road. She had done so and this stupid motorcyclist hadn't stopped too. My workmate asked his wife, "Did either of you consult the motorcyclist?". Result, doghouse!
Hidden danger - hillman
My workmate told me that his wife was very indignant with a motorcyclist who had collided with her car. It transpired that another lady had signalled her that she would give way and let my workmate's wife out of a side road. She had done so and this stupid motorcyclist hadn't stopped too. My workmate asked his wife, "Did either of you consult the motorcyclist?". Result, doghouse!