Joe Bloggs did a training flight with me yesterday. At the end of that flight he still needed one hour of PIC time to meet the requirements. He flies that hour today, but I now cannot revalidate his rating so he has to go on the usual examiner hunt before his rating expires.
It's a silly restriction that severely limits the utility of the "obligation". What's the point of introducing something intended to reduce the bureaucracy of the revalidation process only to trammel it with a restriction that makes it unlikely to be used?
When I applied for it, I did not interpret "upon completion of the training flight" as meaning "immediately following the completion of the training flight". Had I done so, I would have realised that the "obligation" would be almost useless and I would have saved the £53. My mistake.