At six, Hannah Clark received a heart transplant...but they kept the second heart in her chest as well. Ten years later, her new heart failed and they were able to remove it because her old heart had mended itself in the meantime.
For the first few months, the new heart did nearly all the work, but this "rest" gave Hannah's own heart a chance to begin recovery. By the time of the second heart was finally removed, Hannah's heart was doing virtually all the work itself.
It's amazing what doctors can entice the human body to do.