export to camera... "dubbing protect"?!

I have made lots of iMovies, finishing with File--> Export. This time, I put together a bunch of clips imported from past iMovie projects, did some Effects and Transitions as usual on this iMovie 3.0.3 and then hit Export. After the red recording light lit for a couple seconds, "dubbing protect" flashed a few times and the red light went out, while the project continued to play through to the camera...? (Yes, the little tab was closed on the cassette ;-) I tried different cassettes and cameras, with the same result. Other projects still Export fine.