Deleting the last branch on a page will redirect you to a non-existent page (BB-10656)
I was removing all the "merged" branches. When deleting the last branch on ?page=2 I then got a 404. Obviously changing the page to 1 worked.
Ideally I would have been redirected to the next lowest page. (3 would become 2) But redirecting to page 1 would be sufficient at the least.