On a clean installation of current SVN (r9409), phpbb_chmod produces the following warning:

[phpBB Debug] PHP Notice: in file /includes/functions.php on line 581: Undefined variable: file_uid

The warning appears on every page where phpBB tries to access a cache file which doesn't exist (e.g. in case the cache is empty). When calling the same page a second time, the warning no longer appears. However, after having purged the cache, the warning shows up again.

When taking a look at phpbb_chmod, the variable $file_uid is indeed undefined in line 581 in case the preceding chown call (line 567) has failed.