If I over-voltage Side-A of a Si86xx Digital Isolator, what happens to Side-B?
During an over-voltage failure on either side of the isolation barrier, there will be damage to the internal circuitry only on that side of the isolation barrier.
This is to say that if an over-voltage failure on Side-A causes a short to ground or power, there will not be failure or short on Side-B. Side-B will continue to operate as defined in the datasheet, which most often means returning to its default outputs.
The situation is the same when Side-B experiences failures and Side-A does not: Side-A functions according to the data sheet, where Side-B is listed as UP in the "Device Operation" section.
Knowing what happens in such a scenario can be crucial for fault-detection systems!