Page-specific PHP errors usually mean conditional code paths are failing.
Shortcodes, page templates, or custom queries often execute only on certain pages.
Enable debugging and reproduce the error on the affected page. Look for undefined variables or assumptions about global state. WooCommerce and custom post types commonly trigger this when expected data isn’t present. A common oversight is testing only the homepage after changes.

The takeaway is to test all page types when modifying PHP logic.