When is consolidating two competing pages the wrong call?

Two pages competing for the same query is the standard justification for a merge, and most of the time the merge is right. The cases where it is wrong are the ones worth naming.

The first is when the two pages serve different stages. A comparison page and a product page can rank for overlapping queries and still both be needed, and merging them produces one page that does neither job well.

The second is when the competition is a symptom rather than the problem. If internal links point at the weaker page, the pages are not competing. One of them is being promoted over the other by the site itself, and fixing the links can resolve it without losing a page.

The third is scale. Merging two pages is a decision. Merging four hundred is a policy, and a policy applied to a set nobody audited individually removes pages that were quietly working.

My rule now is that a merge needs a reason beyond the overlap itself. What is on your list of reasons not to merge?