Transactions document says:

1. Commit timestamps cannot be set in the past of any read timestamp that has been used. This is enforced by assertions in diagnostic builds, if applications violate this rule, data consistency can be violated.

2. The commits to a particular data item must be performed in timestamp order. Again, this is only checked in diagnostic builds and if applications violate this rule, data consistency can be violated.

Though documented I couldn't find these checks/asserts in the code.