Today, the validation binary returns multikey errors as an exhaustive list of affected documents. While this is handy, it can trip size limits for downstream users of validate() if there's tons of affected namespaces. These error messages add up to over 16MB in some cases.

Index a_1 is not multikey but document with RecordId(20) and _id: ObjectId('580e13a64a33cc49d6123456') has multikey data, 1 key(s)"

...

Index a_1 is not multikey but document with RecordId(200) and _id: ObjectId('580e13a64a33cc49d699999') has multikey data, 2 key(s)"

Instead of a list of documents, let's consider a single error per namespace

Index.*is not multikey but has documents with multikey data. For more information, see log id xxxx"