When you upgrade DNN sites, it usually starts with you extracting a ZIP-file on top of you installation, which is great if the ZIP-file contains all the files in your installation, but what happens in case it doesn't? Like if old functionality in previous version of DNN has been replaced with new functionality, that does not share the same filenames? Will the DNN installer do any kind of housekeeping or are they just left in the installation.

In most cases, this wouldnt be a big problem, as those files would not be referenced any more, but what if, for example, one of those files contained a know security vulnrability and it happend do be a .aspx file? Then that security vulnrability would still be there, accessable from the Internet, even though you have upgraded to a DNN version which claims to have fixed it.