backendError 500 (Bug in nextPageToken) - google-drive-api

Question

For some of the users iterating through the list of files started producing error 500 (backendError). Very few cases, but still.
Close investigation showed that sometimes after requesting a list of files strange nextPageToken was returned. Here is an example:
!!|~EAIaggELEgA6egpg96K5Moz_____f_8AAP__AAD_7qNsfh7Iy8rJyMnDy8bLytHI0bDOy6W4rZ6wvqmc_wAA__8A_v_-f_8A_wDuo2x-HsjLysnIyc7LxsvK0cjRsM7LpbitnrC-qZz_AP_-EJzHASFVhA7Kvoa7STkAAAAAc81GXUgBDEAAIgsJ4YZTXBEABAAgBg
This part right here "!!|~" (start of the string) was causing the errors. Removing it fixed the issue.
There were not so many cases. But still, can this be a bug?
UPDATE 1.
Here is a code sample using Google APIs Client Library for PHP. But the error reproduced only for few users. So there are not a lot of chances of reproducing it.
$client = new Google_Client( );
// ... setup client
$driveService = new Google_DriveService( $client );
$parameters = array();
try{
do{
$files = $driveService->files->listFiles( $parameters );
$parameters['pageToken'] = $files->nextPageToken;
// ...
} while( $parameters['pageToken'] );
}
catch( Google_ServiceException $exception ) {
// ... handle exception
}
}