Debugging customizations is harder than it needs to be because messages from gpc_api.php do not identify the parameter causing the failure. This can be fixed by:

a) changing the relevant error strings in strings_xxx.txt to accept a parameter, e.g.,
change:
$MANTIS_ERROR[ERROR_GPC_VAR_NOT_FOUND] = 'A required parameter to this page was not found.';
to:
$MANTIS_ERROR[ERROR_GPC_VAR_NOT_FOUND] = 'A required parameter to this page was not found: %s.';

b) changing the functions in gpc_api.php to use error_parameters before trigger_error to add the parameter name, e.g.,
error_parameters( $p_var_name );
trigger_error( ERROR_GPC_VAR_NOT_FOUND, ERROR );