If you have Kompozer open and try to open a file in Kompozer by right clicking and choosing "Open with > Kompozer" a blank Kompozer window is launched instead of the file.
If you quit Kompozer and then launch the file with "Open With > Kompozer", the file opens as expected.
This happens on Mac OS. I've not tested on other OSes.
I have this problem as well, using Mac OS X 10.6.8 and Kompozer 0.8b3