I have found that heaters are one of the more common culprits, and need to be regularly replaced. If not they can get worse over time, and without warning "you" can end up with a large enough shock to stop your heart, when in combination with the other culprits when you put your hand in the tank.
You can usually notice the fist signs of stray voltage if you have a tiny cut on your hand and do a water change. That's when I start unplugging stuff, and replace it when found.
I am incredibly surprised that my finnex controller was going nuts. My 3 aqueon heaters are less than 6 months old which is frustrating, so is my powerhead. I am looking into new heaters and temp controller. May just skip the powerhead at this stage.