Insert into a collection a document with duplicated fields like this: {_id: 0, bad: {c: {obj: 42}, c: [42]}} (note: I don't know how to do it from the mongo shell but it looks like mongocxx driver can do that, at least, this is what I'm observing when using Genny that uses mongocxx). coll.createIndex({"$**": "columnstore"}) -> triggers an invariant in ColumnShredder::appendToArrayInfo (and, therefore, crashes the server).   invariant(oldIt < oldPosition.end()); oldPosition = {_data = 0x5651220464b0 "{" , _size = 1} newPosition = {_data = 0x7f44d33e5df0 "{[" , _size = 2} std::mismatch returns newIt set to "[" and oldIt set to oldPosition.end()