iconv() Detected an illegal character in input string

[eluser]Ki[/eluser]
Noticed that logs are filling up with a message from the same ip:

ERROR - 2011-01-19 12:13:23 --&gt; Severity: Notice --&gt; iconv() [<a href='function.iconv'>function.iconv</a>]: Detected an illegal character in input string /home/site/public_html/ci_sys/core/Unicode.php 89

The weird thing is that it is coming from the same ip. I can see that it is a user and not a bot because I see them passing CAPTCHA pages, doing updates etc. No malicious actions.
The error comes up on all pages that user goes through, regardless if they submit $_POST variables or not.

ANY help?