This is hard to blame on a CRO as the sponsor should have oversight. Every trial I have worked on as a separate unblinded person (in the company or consultant) who doublechecks the vendors's(CRO) treatment assignments.
Randomization and treatment assignment mishaps are the most important aspect to control and have good oversight on because they can completely ruin a trial. You can be hands off in other areas if you are low on resources, but not in this area.