In the real world, what happens if a transistor turning on causes it to turn off?
If I understand correctly, you are trying to show the below circuit and want to know what happens if the user raises the user control.

What happens will depend on which component gives up first.  The devices are each trying to maintain certain states, but they conflict, so they cannot all win.  In theory, we can write equations for each of them, but you asked real world, so I'll speak in generalities here.


The bottom transistor is trying to turn on hard.  About the best it can do due to the way a BJT works is to pull the collector to about 0.3 V of the emitter.  This is called saturation.
However, the top transistor wants to keep it's base to emitter to about 0.7 V.  This is very similar to how a diode operates.  As the base to emitter voltage is forced larger, the current will rise exponentially.
The voltage source for the power supply will attempt to maintain the voltage across it.  But in real life, power supplies have limited current capabilities.


In a real circuit, the exponentially rising current across the upper base to emitter will either pull the power supply down if it cannot handle the current or the heat generated across the transistors will destroy one of them.



2