WiredTiger saves a reference to the last item returned from a page in order to speed up returning the next/prev item from the page (for example, row-store items might be prefix-compressed and having the previous expanded return key makes expanding the next key faster, or column-store items might have an non-zero RLE, and the previous return value will be the same as the current return value).

Under very rare circumstances when a cursor crosses a page boundary, the saved information from the last entry returned from the previous page can appear to be adjacent, or equal to, the first entry returned from a newly instantiated page, in which case we can return the wrong key/value information, thinking we have useful information cached.