I also observe this issue in multiple Windows OS'.  Below is the offending code:

echo "Modifying $file\n";
				$currentFile = str_replace("c:","C:",$testFileDirectory) . "\\" . $file;
				$fh = fopen($currentFile, 'r') or die("can't open the original file");
				$tmpfile = getcwd() . "\\temp\\" . rand();
				$fp = fopen($tmpfile, 'a') or die("can't open tmp file");
				$temporaryString = "";
				while ($tmpStringData = fread($fh,1024))
					{
						$stringLength = mb_strlen($tmpStringData);
						$coinFlip = rand(0,1);
						switch ($coinFlip)
						{
							case 0:
							
							while (mb_strlen($temporaryString) < $stringLength)
							{
								$temporaryString = $temporaryString . rand(0,9);
							}
							fwrite($fp,$temporaryString);
							break;
							
							case 1:
								fwrite($fp,$tmpStringData);
							break;
						}//end switch
					}//end while
				fclose($fh);
				fclose($fp);

				rename($tmpfile,$currentFile) or die("Unable to rename the tmp file.");

				$fileToMD5 = getCWD() . str_replace("c:","C:",$testFileDirectory) . "\\";
				addMD5($currentFile);

The error occurs during the rename() after fclose().  It seems that the file is locked or otherwise prevented from being renamed.  Perhaps fclose() is lagging somehow?  I have verified that the file does, in fact, exist when this issue occurs.

This bug has been fixed in SVN.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.