When concurrent transactions fail with different storages (e.g. only network
issues between C1-S2 and C2-S1), in such a way that each transaction can be
committed but not both (or the cluster would be non-operational), and if the
first transaction is aborted (between tpc_vote and tpc_finish), then the second
wrongly failed with INCOMPLETE_TRANSACTION.

And if both transactions could be committed (e.g. more than 1 replica),
some nodes would be disconnected for nothing.