Unless we start testing the actual DAO methods, I don't see an immediate way to test this. What I meant with my previous comment was:

Prefix:
When we delete a client we perform 2 queries. 1 delete in the Clients column family and one delete the column whose name === clientId in the ClientsByUser Column family for the user who owned that client. Because of this bug, nothing would've been deleted in the ClientsByUser column family, as we were using a timestamp as the column name, rather than the ID.

When we're getting the clients for a user, we retrieve all the client IDs associated to that user (back then, in the column's value). Then we do a multi-get in the Clients column family and return.
If a client was deleted it would still have been in the ClientsByUser set. However, it would be filtered out in the multi-get step in `_getClientsByIds` as we don't convert rows into clients if there is no data there.