short circuit with two power supplies
Power supplies are (usually) voltage sources, approximating ideal voltage sources, so they have a very low impedance looking in. A nonideal voltage source can be approximated as a resistance in series with an ideal voltage source. If we connect two such sources in parallel, we get the following situation, which reduces to a simpler equivalent circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

So if \$V_1\$ and \$V_2\$ are not precisely the same, a nonzero difference voltage \$V_1-V_2\$ is applied across \$R_1 + R_2\$. Since \$R_1\$ and \$R_2\$ are very small (ideally zero), even a small \$V_1-V_2\$ can cause a large current to flow.

If \$V_1 \neq V_2\$ and \$R_1 = R_2 = 0\$, then there is a pure short circuit: the circuit cannot be solved for current flow due to division by zero in \$I = V/R\$.