This is one of the known bugs. Once this problem occurs, the HR20 will not record anything correctly until a RBR is performed. After the reboot, everything is normal again. It happened to me as well. Fortunately, I noticed it before too many recordings were affected.
I have never done a rbr for this. I either just delete it, and all is fine after, or two days ago, I said don't delete, and hit the Keep Until button, then went back, and it played for me.
Last night I did have a new one I have not seen, hit play from the list, then play again, and it just froze, hung , like a windows PC. THAT needed the rbr, it was totally hung out to dry. And then after the rbr, I lost nothing, and played the same movie in fact, Phantom Menace.
But on the delete prompt when I hit play, I have never done a rbr and all was fine. Sometimes I recover by puttsing around, and can then play it, else I just delete it and carry on. That being said, its only happend to me probably 5 times in the 6 weeks I have had the hr 20, so mine may not be a good measure to go by.