The web service has the option to batch submit, for example adding ratings to many recordings at once. However, when one of these entries is invalid (e.g. the mbid for a recording no longer exists), the entire batch (single web request) fails with the unhelpful "400 bad request" reply. Since you can never be totally sure that the data you submit is valid (an id can always get deleted before your request), this almost forces you to use a one-by-one approach on submitting, making batch submitting not really an option when you are unsure of the validity of the data (this problem increases when using "older data sources", for example the ID3 tags in MP3 files. Additionally, the unhelpful error makes it hard to diagnose potential problems when developing a program that uses the web service.

I think there should be an option to batch submit stuff that allows for some invalid entries. Missing/invalid mbids can be ignored, or even better some kind of status is reported (a list of failed mbids for example, though I'm not sure how technically possible this is). The correct entries get submitted in this option (do not fail the entire batch when even a single entry is wrong).