When a member of a relation is deleted in Josm AND the relation is open in the editor, josm will give a conflict when closing the relation editor.
I got this several times while editing a network relation.
The network relation was open in the editor because I was adding route relations to it.
This relation did include members (nodes, ways) which were deleted by me because they were incorrect (duplicate ways, without tags).
When closing the relation editor, I get a conflict.
That’s right: When you’re deleting a member of a relation this means you’re editing the relation (to remove it). When you’re closing the relation editor the changes are applied (not before!). So when you’re deleting a member of a relation while this relation is opened in the relation editor the deletion is not applied in the relation editor, but in the dataset. When you now save the (also changed) version from the relation editor, you’ve got two states which are not subsequent, called “a conflict” (same will happen if you upload data someone else changed while you were editing, which is usually a lot harder to resolve).
If you also applied the change in the relation editor manually you can easily resolve the conflict by completly choosing the right (as in “not left” as well as in “correct”) side in the conflict manager.