Two parallel wires with no resistance - why it's wrong?

Two parallel wires with no resistance - why it's wrong?

This is wrong because there are no wires with no resistance. When trying to resolve Kirchhoff equations for this loop (or whatever their algorithm is), the simulator would run into division by zero exception. To avoid program crash, it likely analyzes these conditions and declares the circuit as error. Try to replace wires with resistors with micro-Ohms value (or maybe even less), it should be fine.

Falstad wants to be able to animate the circuit, showing how much current flows in each wire.

In this circuit, that isn't possible, so it is indicating this as an error.