Inline edit of multi-record custom fields broken

I just tried this out for the first time and found it to be fairly broken, not sure if it's a regression or if it was just never tested.
To reproduce:
Edit contact the old fashioned way and put in more than one record for a multi-record fieldset.
Go back to the view page and try to edit those records inline - saving the form for record #2 will instead update record #1, then editing record #1 won't work at all.
Since multi-record custom is pretty much the same as address, I'll look into reusing that code.