Question: If I feed a duplex receptacle from two separate branch circuits, I'm supposed to remove the jumper tabs between the receptacles. But what would happen if I didn't remove the tabs?

Answer: 
With a 120/240V single split-phase system, there are two possible outcomes.

Separate legs
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If the branch circuits feeding the device are on separate legs of the service, then the tab will be creating a direct short-circuit between the legs.

[![Service legs direct short-circuit](https://i.stack.imgur.com/RP9XX.png)](https://i.stack.imgur.com/RP9XX.png)

This will cause a high current through the circuits, which should trip one of the breakers fairly quickly.

Same leg
========

If the receptacle is fed by branch circuits on the same leg of the service, the danger is a bit more subtle. Initially everything works fine, no sparks, no tripped breakers. In fact, this situation can go unnoticed indefinitely, and never cause a problem.

The problem only occurs, if you try to draw more current than the individual branch circuit ratings. In the example below, the branch circuit protection for each circuit is 15 amperes.

[![Split receptacle same service leg.](https://i.stack.imgur.com/VtNve.png)](https://i.stack.imgur.com/VtNve.png)

If you put a load on the circuits, you'll find that the current will take all available paths. This means that the current will actually be divided between the circuits, so that each will see half the current that's flowing through the load. The yellow highlighting below, depicts the electricity's path through the circuits.

[![Electrical flow through the circuit](https://i.stack.imgur.com/dE0iq.png)](https://i.stack.imgur.com/dE0iq.png)

Still, there's no problem here. As long as the load does not draw more than the circuit rating, everything chugs along just fine.

The effects of this mistake only become evident, if the load draws more than the circuit rating. If the load in our example draws 30 amperes, you'll start to see the problem.

[![Load drawing over branch circuit rating](https://i.stack.imgur.com/W0WLN.png)](https://i.stack.imgur.com/W0WLN.png)

If you measure the current flow at various points along the circuit, the problem becomes obvious. Each branch circuit only sees 15 amperes, so neither breaker should trip. However, the receptacle will have to handle the full load current, which is well beyond the rated current. The receptacle will heat up and fail, potentially starting a fire as it does.
If you add two live 15 amp lines to one outlet you will blow the breakers if not the main.