Suppose you're using TablePager to paginate some data with few unique columns. For instance, let's say that our table has mytable_id (unique), mytable_name (not unique) and mytable_date (not unique), and you want to allow sorting by name and date. You cannot do this by simply overriding isFieldSortable because the fields are not unique. According to the documentation in IndexPager::getIndexField(), the following should work:

public function getIndexField(): array {
	return [
		'mytable_name' => [ 'mytable_name', 'mytable_id' ],
		'mytable_date' => [ 'mytable_date', 'mytable_id' ],
	];
}

but it doesn't: it always uses the first element (mytable_name), regardless of what the actual sorting is. This was also discovered and reported by @matej_suchanek at T191694#6469506. This seems to be caused by an inconsistent naming in TablePager vs IndexPager, since the former uses "sort" but the latter uses "order".

AbuseFilter has an example of this, and we have another in the CampaignEvents extension.