Investigate what allow_multiple is supposed to mean and whether it has an effect. Test and document the reality. This about parsing a buffer like:

{"a": 1}{"a": 1}{"a": 1}

Read the YAJL documentation to see what it's supposed to mean.

Could be that our error-handling code recovers when allow_multiple is false but we detect multiple docs, causing libbson to act as if allow_multiple is always true.