It is possible many of the problems reported on this forum may be caused by unknown interactions between different plugins.
I had reported a cross-domain login problem seemingly caused by domain mapping, because it was perfectly correlated with whether or not I added domain mapping to a sub-folder.
Now, I learned the real cause of the problem was a Kaltura.com video plugin I had installed (and now have removed). I tracked down Kaltura as the cause when the YouTube mappings in my posts kept getting erased. The reason showed up in the page source code. Kaltura caused more than one problem. With Kaltura removed, domain mapping works perfectly.
So, while it may be impossible for you to protect your plugins from disasters like Kaltura, this post is to warn readers that Kaltura is bad news.