*A <ref/> tag with no body anywhere in the article should have some sort of glaring red warning to that effect, in the same sort of manner as we see when a <math> tag is fed badly formatted TeX. The current system produces a blank reference, which is better than nothing but still kind of easy to overlook. *A <ref> tag whose body is defined but not actually used anywhere in the article should still appear in the <references/>-generated list, with a bullet instead of a number. This is already done manually in a lot of cases when people add references directly to the references section.
Both good ideas.