"No space left on device" error when writing certain session filenames

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http://serverfault.com – I have some filenames (session files) that can't be written ("No space left on device"). Other filenames (same directory) are fine. Disk is not full. The filesystem is ext3 PHP is giving an error when trying to create the files, but the error can be reproduced on the command line: # less /path/to/session_data/sess_u2q1pfelfr0jof3mp38jb2eaj1 /path/to/session_data/sess_u2q1pfelfr0jof3mp38jb2eaj1: No such file or directory # touch /path/to/session_data/sess_u2q1pfelfr0jof3mp38jb2eaj1 touch: cannot touch `/path/to/session_data/sess_u2q1pfelfr0jof3mp38jb2eaj1': No space left on device # touch /pa (HowTos)