In Views UI Advanced settings, Contextual Filters are placed above Relationships. The problem with this order is that some contextual filters (arguments) only show up if the users chooses a relationship first.
People tend to work top down, so imho some misconceptions about contextual filters and relationships could easily be avoided by changing the order of this items and listing relationships above contextual filters.
In others words, contextual filters may depend on relationships, so the latter ones should be listed first.
