An update operation writes the modified document's documentKey to the o2 field of its oplog entry; this is used by $changeStream to look up the document in the cluster if the updateLookup option is specified. An update on a sharded collection will write the shard key plus _id to the o2 field; on an unsharded collection, just the _id. But this means that if an unsharded collection is subsequently sharded on a key other than _id, the updateLookup for all pre-sharding update events will attempt to target the lookup by _id alone, will be unable to target a single shard, and will therefore always fail with an exception.

It is possible for this failure to occur in one of two different ways:
1. An assertion which fails just the change stream, which would look something like this:

{
  ok: 0
  code: 1,
  codeName: "InternalError",
  msg: "Unable to target lookup query to a single shard: {query.toString()}
}

2. An invariant failure on a mongos process which would look something like this in the logs:

Invariant failure shardResult.size() == 1u src/mongo/s/commands/pipeline_s.cpp