In some access logs, a lot of 408 status response codes are found, and AWStats handles them as 'corrupted log lines'. A typical logged 408 response looks like this:
host:80 184.108.40.206 - - [13/Oct/2013:06:29:03 +0200] "-" 408 0 "-" "-"
Note that where you would normally see "GET / HTTP/1.0" or similar, this requests is logged as "-" as no GET, POST or else was received from the client.
I think AWStats does recognise 408's where the client did get to send an actual request, but a timeout occured after that.
This has been reported several times before but those where closed because of old age (https://sourceforge.net/p/awstats/bugs/search/?q=408). A recent bugreport was submitted here: https://bugs.launchpad.net/ubuntu/+source/awstats/+bug/908901