Edit: OK so it's not the light. But it's a ground fault somewhere. One presumes it is in the circuit, but grounds can be crossed over amongst circuits in the same panel, especially in a ground retrofit situation. So it could be anywhere wired which feeds the hot ground wire.
There should be no intentional current whatsoever on a safety ground. (though there can be micro-currents due to coupling or the tiny, tiny amount of leakage which is allowed.)
Unexpected currents like this are why grounds must be pigtailed, so you don't have to sever a ground to replace a device.
Obviously, bootlegging neutral from ground is the picture postcard definition of a ground fault.
If your previous homeowner did it one place, good chance he did it lots of places.
It's hot because when you interrupt/sever a current return, current is no longer able to return. Normally it flows freely back to the panel, and that pulls it down to near earth voltage. But current can't go any further so it's "piling up" right there.
That saying is wrong. It's dead wrong. The people saying it are idiots. Or you misunderstood.
Imagine if you found an SE post that started out "People often say 'humans do not contain blood'. I think that's a false statement." It's like that.
Neutral certainly does carry current. In a single-phase, 2-wire circuit with neutral, neutral carries exactly the same amount of current as the hot. Neutral is the normal current return.
Now in split-phase or 3-phase where 2+ hots and neutral are brought along, it's likely most of the load will be phase to phase, with neutral carrying only imbalance current. In multi-wire branch circuits, all loads are phase-neutral and neutral acts as a broker to try to return current via another load rather than bringing it back to the panel itself. But neutral is still a normal and legitimate carrier of current, and it counts.
A great deal of work is done to assure that Equipment Safety Ground is at roughly the same voltage as the dirt surrounding your house. That way if you are touching a grounded receptacle while standing on your wet grass in your bare feet, nothing happens.
We also make very strong effort to make neutral reasonably close to ground potential. Nobody cares if neutral is 7.3 volts away from the dirt in your lawn. We just want it to be close enough to be safe.
It's true if you are standing barefoot on your lawn and you touch neutral, you shouldn't be shocked. But that's not the main reason to do it. The main reason is to assure that your "hot" wires are only 120V from the dirt in your lawn. Consider the alternative. Imagine some leakage in the transformer caused your supply to float 2400V above where it should be: Hot 1 would be 2160V from earth, neutral would be 2400V from earth and hot 2 would be 2640V from earth. Now suddenly, touching any wire with your bare feet would be instantly lethal.
By providing a firm neutral-ground bond, in one place, we assure that neutral will be near 0V from earth, and the hots won't be terribly far away either.
Picking any supply wire and bonding it to earth in this way defines that wire as neutral. Neutral need not be "in the middle".
Best Answer
If and only if everything is working
For a theoretical panel, neutral is theoretically safe to touch if:
But then, if all that was true, you wouldn't be opening up your panel, would you? :)
But yes. If things are in order, there will be a grounding electrode system going from ground rods, metal water pipe or Ufer tie-in to your basement's reinforcing rod, which establishes contact with earth proper. There will be one at the transformer tying to neutral, and one at each building's most main panel tying to ground.
Next, in every building the grounding electrode system will be bridged to the service-wide network of green, green/yellow or bare wire, or metal conduit, known as the equipment grounding conductor, or colloquially "ground" in the electrician's context, not to be confused with GND or Vss in electronics, which is why I steer out of my way to call this equipment safety ground. No current ever flows on this except during a fault condition.
Finally, one place in every service (i.e point fed by an electric meter, customer demarcation point, etc.), typically at the service point, has a neutral-ground equipotential bond. That has several jobs. Relevant to your question, it is to do exactly what you are concerned about: clamp neutral so it is near ground and cannot kill you. No current sgould flow on it because no current should be on ground, and transformers shouldn't leak.
By the way, the neutral-ground bond is a great place to put a clamp ammeter. Helps to wire your panel so that is easy.
In a residential setup, you optimistically have several houses sharing a transformer, and the transformer and each house's neutral-ground bonds are all wrestling neutral down to safe voltages. In an industrial setup, that is not the case, and I got a real eye-opener when a transformer's only neutral-ground was not actually attached.
Neutral could have bitten me at any time, why didn't it? I like to credit good practices: I treat neutral and other ”known”-dead wires as if they're live. The takeaway is this:
Neutral is live until you have done a physical inspection that all the earthing hardware is in working order, and done an electrical test to confirm.