Example of what happens when a file like  www.domain.com/wrong_file.pdf  is called using default class.tslib_fe.php :

NB: there variables listed either side of functions show the vlaues of these variables before & after these functions.
    Where the functions are indented further than the preceding function this is due to that function being called inside the previous named function.

in /typo3/sysext/cms/tslib/

     $TSFE->checkAlternativeIdMethods()	// called from index_ts.php # Since the 'TYPO3_SITE_SCRIPT' is not a valid typo3 SSD page. /*.html

     $TSFE->determineId()	// called from index_ts.php

		$this->id = ''
          $this->setIDfromArgV()	// class.tslib_fe.php # $this->id must be 'false' in order for any processing to happen in here, BUT sinve there is no alias $this->id is set to 0
		$this->id = 0

		$this->id = 0
          $this->fetch_the_id()		// class.tslib_fe.php # Get The Page ID
		$this->id = 0

			$this->id = 0
               $this->checkAndSetAlias()	// class.tslib_fe.php # Fetches the integer page id for a page alias.
			$this->id = 0
			$this->pageNotFound = 0

			$this->id = 0
               $this->findDomainRecord()	// class.tslib_fe.php # Looking up a domain record based on HTTP_HOST
			$this->id = 0


               since $this->id = 0 , the next code block starting with the following is executed.

                    if (!$this->id)	{ // from about line 861 in TYPO3v4.1.6 

      *****             after the code block that follows this if statement $this->id = 1

      *****     this is why i suggested to replace the above if statement with this line of code, which stops the processing of $this->id when $this->id=0

      *****         if (!$this->id  && (t3lib_div::getIndpEnv('TYPO3_SITE_SCRIPT')=='index.php' || !(t3lib_div::getIndpEnv('TYPO3_SITE_SCRIPT') && $this->TYPO3_CONF_VARS['FE']['pageNotFound_handling'])))	{


			$this->id = 1
               $this->getPageAndRootlineWithDomain()	// class.tslib_fe.php # Gets ->page and ->rootline information based on ->id. ->id may change during this operation.

			$this->id = 1
                    $this->getPageAndRootline()	// class.tslib_fe.php # Gets the page and rootline arrays based on the id, $this->id
			$this->id = 1
                      // inside this function if $this->id still equals 0 after $this->sys_page->getPage(), and if $this->TYPO3_CONF_VARS['FE']['pageNotFound_handling'] is true then $this->pageNotFoundAndExit() is called
			$this->pageNotFound = 0

				$this->id = 1
                         $this->sys_page->getPage()	// class.t3lib_page.php # If no page is found an empty array is returned.
				$this->id = 1

                           // inside this function; if $this->id starts with a value of 0, then no page is found and an empty array is returned to function getPageAndRootline()


     $TSFE->getConfigArray();	// called from index_ts.php