Breaker off but shock when electric heater kicked on
Well it really shouldn't matter if you have the breaker off but in years past a multi wire branch circuit did not require handle ties or common trip. This may be the case and you turned off 1 breaker but the other breaker was still using the neutral any imbalance will be on the neutral, or the hot on the circuit you thought was turned off gave you a shock. Been there in residential and industrial.  Current code protect for this with a common trip on these kind of circuits.I will need to find my 80' S code books because there was no dedicated circuits back then and if on what we call a MWBC today handle ties were not required. Edit i was an apprentice in the 70' S back when zinsco and FPE was the the best or low cost options avabile did lots of them in my apprenticeship and after returning home.