Originally Posted by stillmarried
When we flip the light on, it causes the GFCI outlet breaker on a completely different circuit (on the opposite side of the panel, even) to flip.
This occurs only if our low powered microwave is plugged in to that circuit (but not even running).
we can reset the GFCI and it stays on, even with light on AND microwave running.
If a grounded microwave, check for leakage current in the ground pin.
So you are looking for some transient event, or the GFCI is running close to its 5 mA limit.
I guess you could try unplugging the microwave, turning on the light and then plugging in the microwave.
There might also be a high resistance connection in the neutral busbar.