In sharded clusters, doing QE writes will fail with the following error:

{ "nInserted" : 1, "nMatched" : 0, "nModified" : 0, "nRemoved" : 0, "nUpserted" : 0, "writeErrors" : [ { "index" : 0, "code" : 263, "errmsg" : "Transaction was aborted :: caused by :: from shard rs1 :: caused by :: Cannot create new collections inside distributed transactions" } ] }

if the ECOC collection does not exist and needs to be implicitly created. This normally happens when the compaction operation renames the ECOC collection to a name suffixed with .compact, but does not create a new ECOC collection as replacement.