Page has no title
The title is the first thing a screen reader announces and the label on every browser tab. A missing one hurts accessibility and search rankings at the same time.
What a shopper experiences
The title is how anyone tells one page from another. It is announced first by a screen reader, it labels the browser tab, and it is the headline in search results. With several tabs open and no titles, a shopper cannot find their way back to the product they were looking at.
Why it shows up
Product and collection pages get their titles automatically from your catalogue, so this rarely affects them. It turns up on custom landing pages and on pages generated by other apps, where nobody filled the field in.
How to fix it
In Shopify admin, open the page and look for the search engine listing section near the bottom. The page title field there is the one. Write what the page actually is — "Size guide" beats "Page 3" — and you have improved your search results at the same time.