With circuit off, why would neutral wires spark and lights on a different circuit flash?
It sounds like you have a neutral shared between loads on two (or more) circuits. This can be hazardous for a couple of reasons.
Firstly because when a neutral that is supplying a load is disconnected it will become live. So you test for dead with everything connected and it looks fine, but then you disconnect the neutrals and suddenly you have a live wire in the box.
Secondly because the neutral conductor may not have adequate overload protection.
There is a legitimate construction known as a multi-wire branch circuit where two separate hots share a neutral, but extra precautions are needed to make it safe. Firstly the two hot lines must have &quot;common maintenance shutoff&quot;, either with a double pole breaker or with the two breakers handle-tied together. Secondly the two hot lines must be on different &quot;legs&quot; of the supply to prevent overloading of the neutral conductor.