tl;dr JSON Patch paths should have / & ~ escaped with ~1 & ~0 respectively, see http://tools.ietf.org/html/rfc6901#section-4 for more details on what's required.

I've submitted a PR to evanphx/json-patch with the fix - will need to update vendored dependencies once that's merged (hopefully soon).

Would like to see this fix in 1.3 unless it's too late?