Scenario, all must be true:
- Relay contacts welded stuck (extremely rare, but possible w/ high-current arcing, spring/metal failure, etc.). This will prevent disengagement of the relay from the grid.
- The only person behind your local service transformer, so if the grid goes down, you're not trying to power the entire neighborhood.
- Grid failure occurs between said transformer and the rest of the grid. Transformer becomes a live step-up to anyone working on the other end.
- Apparent load between inverter and whatever is left of the grid somehow does not appear to be a dead short, which in normal cases, will trip your breaker.
Of course, I have also tested various grid-down scenarios during my installation, which works as expected. For all practical purposes, yes, it is effectively impossible. However, in the above scenario, it is theoretically possible, though also extremely improbable.