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Go to the Altavista free submission page, enter an arbitrary URL and an arbitrary invalid submission code, and submit. In the resulting error message, all seems to be fine, even if you check the sources with Netscape (on Linux, anyway).
However, if you save the file to disk, and open it with a text editor that can handle binary data, here's what you will find:
We're sorry. The Submission Code you typed into the text box did not match the Submission Code in the image. Please hit Back and try again. Or, click <A HBEF="http://addurl.altavista.com/sites/addurl/newurl?url2add= http:_//www.example.com/\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0 \0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0 \0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0 \0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0 \0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0">here</A> for a new Submission Code.
(spaces inserted for wrapping) where every "\0" stands for a binary null-byte, the ASCII value 0x0. I'm pretty sure those are not supposed to appear in valid HTML... ;)