It is pretty normal. In theory it probably shouldn't happen. It hapens more often if the voltage necessary for firing the plug is higher.

It would be likely that the insulation should take 30kv or more. A plug normally has enough resistance in its gap and all the resistors in the wiring to cause the firing voltage to be 10kv or so.

A wore out plug with a very large gap or an open circuited plug end resistor might cause the firing voltage to approach the end of the coils capability at 30kv. if you pull a plug wire and hold it away from the engine the open circuit voltage goes to available voltage, around 30kv.

If at anytime a lower resistance path to ground appears that becomes the new path. When holding the disconnected plug wire it is often common for sparks to jump from the center coil terminal to the ground of the coil/controller (one reason it not good to open circuit secondary wiring).

So if it jumped to you, then the insulation is weaker than the available voltage and you were closer than the plug in voltage potential.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician