Except they "give an error" because the provided field doesn't exist in the database. Ignore for a second that half the reposts would break websites if an unexpected parameter yield an error instead of being ignored, if an untrusted client sent an "id" field, that would go through like hot steel through melted butter.

Well. The likeliest thing is that there is no "middle man fucking with them". The likeliest thing, since it's an iOS app posting to their API, is that they're introspecting a client-side object to get the values they care about. And that they're blacklisting values they know they don't want, rather than whitelisting values they know they do want.

Which meant that when a new property showed up their app blindly submitted it to their web API, and their web API blindly accepted it because it was doing mass assignment, and that's when the API broke.

Which really just hammers home the point people have been trying to get you to see, which is that these types of idioms -- mass assignment, blind trust of client-supplied data, blacklisting instead of whitelisting -- are really serious problems that should not be encouraged, and should not be swept under the rug.