What will happen if you do NOT cut the red wire on an ESC when using a BEC? You would have power from both the ESC's built in BEC and the separate BEC going to the rx. Both powered from the same battery. I'm guessing a diode prevents backfeed into the ESC in such a case.

What negative things might happen if you did not cut the red wire? A friend of mine did this on a helicopter and nothing bad happened that he could notice. Castle creations and kontronik instruct you to clip the wire. His was a castle but it did work without clipping it.

I'm putting Jeti controller and a UBEC in a new plane which is why I was curious what the worse thing that could happen if the red wire was not cut.

Not clipping the red wire will burn up your ESC I know I did it
With my castle ESC it didn’t happen right away but after 15 to 20
Flights it fried

so my buddy has been very lucky so far that he hasn't toasted his castle phoenix 80 thus far! I'll let him know to break out the clippers or pull it out and shrink wrap the red lead

Just pull the pin and wrap it with a little tape to insulate....

Using the UBEC and internal BEC is bad mojo....the UBEC will constantly try to make up for the voltage drop of a linear regulator...