@User to get the reason for the warning open the ERC window:

View->Show ERC window

, then press the '`Netlist`' button. This tells you the reason. Looking at the circuit it seems that the '`vip`' net is driven by the input pin and by the differential amplifier output port. An input pin is considered a driver, you are supposed to apply a stimuli to it or the net is driven by a parent schematic. Since the differential amplifier output port also drives this net xschem issues a 'shorted output node' warning. There are 3 ways to get around this: • leave it as it is, you know the reason. • Modify the differential amplifier symbol output port 'dir' attribute to 'inout' (and corresponding schematic output pin to 'iopin.sym') so no warnings will be issued. • Change the 'vip' pin from 'ipin.sym' to 'iopin.sym. (Recommended action) In all cases the netlist can be simulates without problems.