I think I already saw this somewhere, but I have to emphasize again that it is very annoying once httpd -t says that there is a bad configuration file, that first all custom items are commented out and a "# NOT SUPPORTED" is added at the end of the directives, even though this might not help. In that case I have to go back (probably directly via phpmyadmin) and remove all those not-supported comments. Very annoying, since the actual error was made by me in apache2.conf directly.
And today I also had the "cannot decrease number of domains" message where it somehow confounds the number of domains with the number of users of that web. I don't remember exactly what situation triggered it, but if I do I will report back