Common and acceptable? Yes = and a 'wake up' call for you.
First of all, do not consider that "0 volts" means, by itself, that a wire is 'dead.' Every neutral, and many grounds, will read '0', yet still be carrying plenty of current. Open that neutral or ground, and, suddenly, one end becomes 'hot' - and will zap you bad.
Look at it this way ... the electricity leaves the panel, does it's work at the light, then returns to the panel. You don't want to get across that path .... anywhere in the circuit.
Next, remember that electricity takes ALL paths home. It cares not if something is at the 'end of the run' or not. So, if a circuit is hot, and has more than one light on it ... and you touch the neutral at 'the end' to ground ... some of the current powering the remaining light will return via the ground path you've just created.
Finally, it's possible that the spark was caused by a capacitor, or other electronics, discharging.
Now - here's the "nanny statement:" 277 can easily become a 'can't let go' situation. TURN IT OFF. Sure, that fiberglass ladder might offer you some protection .... but if the shock don't get you, the fall might.
