So I realize I've been lapse on my posting of late. Mostly because I haven't much I'm in a position to share. That said. I was recently running a week of education based on our new curriculum and we (the class and I) ran into a really odd error that we had not seen previously.
Specifically the class was focused on Phasing in Revit. We were going through how you can add a door to an existing wall. In our case, we had already deleted an existing door from the same wall object, and there was an infill portion of new wall where we had removed the door. As part of the class we had changed the wall type of the infill, and we had made it larger then the existing wall. When we placed a new door, the new door took on the dimensions of the new wall infill, and the existing wall was not cut.
With a little bit of playing around, we determined that the door was hosted to the new infill wall, and that by using the "Rehost command" in the options bar we were able to host the door properly to the existing wall. I will warn you that getting the door to rehost properly can be tricky. The behavior of hosting to the new infill seems to be consistent, as students saw the same behavior, and I was able to re-create it again too (admitedly all on the same project file). The training file was started in 2008, before being upgraded. None of remeber seeing this behavior previous to 2009. I do know that Autodesk has had some sample file(s) with this behavior, and they used it as a puzzler at AU2008, however in that case it was a plumbing fixture, not a door.
So, the short is, the behavior is not completely new, but I don't remember it being as prevalent previously. So take this as a word of caution.