Wheel Flop (WF)
It is the tendency for the python not to stretch out, when taking place on the python. In that case. if you steer the front part of balance, the self centering upward force will be smaller then the weight pushing the front part down, causing the front part to slam down onto the ground.
You should only notice wheel flop with very low (badly chosen) pivot angles.
Wheel flop does not make the bike unridable, but it has the nasty effect that taking the feet from the pedals the front part falls sideways hitting the ground with the chainring or pedals. This could happen at a regular stop (cross roads), or when accidentally loosing contact to the pedals (for example riding over a bump) causing a nasty fall.
NOTE: The flevo type bike have a massive wheel flop because of its positive trail. This is counteracted by the rubber block located at the pivot, acting as a spring keeping the front part straight. Experience has shown that removing that block does not impede riding the bike. The author of this text (babypython) even feels that the riding experience was better , because curving wasn't a fight against the spring anymore. In his opinion this shows that excessive self-centering effect has no positive effect on handling of the bike.