Where one net (A) is made up from two wire segments there is an overlapping green start/end point.
If a third wire on a different net (B) then crosses this point an unintended join is made.
If the junction point of the two wires on net A is moved away from the crossing point of the third wire net B, then no join is made.
This video shows exactly what the problem is:
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
