Small dc motors like the one below are commonly used for various tasks in toys and small appliances.
These motors almost exclusively have perfectly cylindrical round shafts without any notches that have an almost polished appearance. The products making use of these motors usually have small plastic gears or corkscrews attached to these shafts which are held by friction alone.
These friction fit parts are usually the first thing to fail in these products which can usually be heard by the motor running on maximum rpm seemingly without making contact with the gear any more.
Why don't these motors have a notch in their shaft to ensure a safe connection?