When it happens
The request has one or more problems, and theerrors array lists them all, so you can fix everything in one pass. Each entry has a code, a detail, and either parameter (the query parameter) or pointer (a JSON Pointer into the batch body, such as /ids/3). At most 20 entries are listed; when there are more, detail says how many there were.
What to do
Read every entry inerrors, fix the request, and send it again. Retrying it unchanged returns the same error.
Example
Body