It's happened to me a few times, actually. As it happens, I was rescued once as the Bohemians as well, although in my case it was the Hungarians that bailed me out. We were faithful allies to each other throughout the game -- I dealt with the HRE, while the Huns held off the Cuman hordes that threatened to overrun central Europe. We actually came to each other's aid a couple times, as we seemed to always manage for one of us to be in a strong position while the other was somewhat weak.