I absolutely agree. I am a firm believer in proper execution, even if it means taking longer than expected. Therefore, going back to a previous phase for any legitimate reason is justified in my opinion. A legitimate reason, however, means that there was no simple way to avoid reverting back. This means that the PM acknowledged the schedules of the key members of the project, reviewing lessons learned for similar projects/related tasks, and properly estimating timelines/costs. In this case, it appears that the availability of critical staff members should have been considered prior to the implementation phase. Although every situation is unique, I believe this was a situation that could have been avoided.