avatars.php in server error logs

FYI: The following error appears quite often in my server error logs …

[Sat Aug 29 10:57:45 2009] [error] [client] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/tripawds/public_html/wp-content/mu-plugins/avatars.php:96) in /home/tripawds/public_html/wp-includes/pluggable.php on line 865

I am not too concerned because ip address lookup reveals the error only seems to be thrown by various web crawlers and search bots.

Just wanted you to know, and confirm whether or not this might indicate an issue I should be concerned about. Thanks!