Spontaneous byte change in .gz file

The Visitors run I do over all my web log files automatically started complaining about one of the compressed files today:

  gzip: /var/www/www.asjo.org/logs/access_log.103.gz: invalid compressed data--crc error
  gzip: /var/www/www.asjo.org/logs/access_log.103.gz: invalid compressed data--length error

Logging into the machine and testing the file confirms the problem:

  $ gzip -t access_log.103.gz 
  gzip: access_log.103.gz: invalid compressed data--crc error
  gzip: access_log.103.gz: invalid compressed data--length error

So I get a copy of the file from backup; it is the same size, and it isn't invalid according to gzip -t. Let us compare them:

  $ cmp --print-bytes access_log.103.gz access_log.103.gz_from_backup
  access_log.103.gz access_log.103.gz_from_backup differ: byte 189783, line 646 is  40    41 !

One byte changed! Yikes.

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