Let's say I perform an edit on a page, and I want to provide a link to someone that shows what I've done. I click on Old Revisions, I tick the checkbox next to my edit, and immediately below it I also tick the checkbox next to the previous edit. Then I click "Show differences between selected revisions".
This shows me a side-by-side diff of the page, highlighting what I did in my edit. I wish a link to this, so I click on "Link to this comparison view", and I get the same page, but with a full URL I can use. Anyone I send this URL to can now see the difference between what was on the page before I edited it, and what was there after I edited it. Right?
Except that is not what the URL shows. It shows the difference between what was on the page before I edited it, and what is on the page now. If anyone edits the page after me, their edit will also be included in the diff. This is because the URL has no parameter specified for "rev2".
This is not a problem when you go through the same process to show a diff between two older, historical, edits. The created URL always has a second "rev2" parameter. It is only a problem if you want a diff with what is the latest revision at the time you create the link. It always assumes you want a diff between an old revision and the latest revision, but what I want is a diff between two definite, specific revisions, one of which just happens to currently be the latest.