Right now if you look at the side-by-side diff, the current revision is on the left, the previous revision is on the right. Usually when side-by-side diffs are presented (such as in every diff software I've come across) it's the other way around. This is confusing to newcomers to Redmine -- I just had an instance now where my friend I'm working on this project with thought that I had deleted something, when I'd actually added it, and it took a second for us to realize that the order was misleading (I guess he didn't notice the colors).

Attached is a patch that corrects this (or appears to, as well as I could read the lib/redmine/unified_diff.rb file).