If the panel has grid-power, the two waveforms would not match up, and the inverter would go poof.
If the panel does not have grid-power because of a power blackout, you are energizing a line that runs out to the street pole, and you are creating a life-threatening situation to a lineman trying to restore power.
So, never do this without the proper either/or switching breaker that isolates the inverter from the grid.