With a Lamport Clock (counter, logical clock) you could end up with the following due to dependence on conflict resolution aka optimistic MVCC rather than Wall Time (copies do from another HN):

“it is possible for a transaction C to be aborted because it conflicts with another transaction B, but transaction B is also aborted because it conflicts with A (on another resolver), so C "could have" been committed. When Alec Grieser was an intern at FoundationDB he did some simulations showing that in horrible worst cases this inaccuracy could significantly hurt performance. But in practice I don't think there have been a lot of complaints about it.”