Start Rosegarden, and another JACK transport client such as Ardour. Ensure they are both synced to JACK.
Press Play (or record) from the "other" application. RG's transport will start rolling, as it should. But when the end of RG's composition is reached, it will stop the transport -- even though the other application may be still playing (and, worse, recording).
If you didn't actually press Play in Rosegarden, then Rosegarden should never stop the transport on your behalf, unless you actually pressed Stop.
(reported by Mike Taht, who lost a recording session)