Bad plug cap?

[ccs]

Forgive me if this is a lame textbook question.

While my '79 RM125 was running I got a decent shock when I touched the boot that snaps over the plug.I've done that a million times before,getting weeds out or for whatever reasons I may have needed to spin it more center and I've never been shocked.

Is that normal,and I've just always been lucky in the past?Or is that a sign of a bad cap?And if so,what kind other problems can it cause?