⚡ Quick Fix (TL;DR)

The Culprit: One snippet calling a function defined in a separate snippet creates a hard, unguarded dependency. If the other snippet is ever deactivated, temporarily emptied for testing, or simply fails to load first, the calling snippet throws a fatal error the instant that function gets called – confirmed directly via a real error log entry, not a hypothetical.

The Fix: Wrap any cross-snippet function call in a function_exists() check. If the function isn't available for any reason, the page simply renders without that piece instead of crashing outright.

if ( function_exists( 'your_external_function' ) ) {
  echo your_external_function( $some_argument );
}