If there is a partial update to a document where the size of the delta plus the size of the document post image is greater than 16 mb, changestreams that request fullDocument:'lookup' will need to be able to handle that case gracefully.
If the final notification entry that would be returned to the user is greater than 16mb, the changestream should be closed and an error should be returned to the user. If a later stage in the agg pipeline projects things out so that the final document returned to the user is less than 16mb, even if the doc was >16mb at some point during the pipeline, then the notification should be successfully returned to the user.