We ran into an issue with Phoenix where a RegionObserver coprocessor intercepts a scan and returns an aggregate (in this case a count) with a fake row key. It turns out this does not work when the ClientScanner encounters NSREs, as it uses the last key it saw to reset the scanner to try again (which in this case would be the fake key).
While this is arguably a rare case and one could also argue that a region observer just shouldn't do this... While looking at ClientScanner's code I found this logic not necessary.
A NSRE occurred because we contacted a region server with a key that it no longer hosts. This is the start key, so it is always correct to retry with this same key. That simplifies the ClientScanner logic and also make this sort of coprocessors possible,