Seems that users who install the latest version of the Google search bar are finding that Google is hijacking a servers 404 error page.
Google grabs the 404 error code returned to the web browser in certain situations and instead of displaying the 404 error page of the website you are on, it creates a custom 404 error page – made by Google. The “new” 404 error page ‘conveniently’ includes a Google search box and if used by a visitor will drive the visitor away from your website. Even worse – the search box is pre-populated with data from the initial URL query on your website. Imagine a situation where kind of sensitive data is send to Google that way. Even normal data would make it to Google that way and we all know what Google does with data. For me as a webmaster this is a major intrusion into my own intellectual property.
In the wild west of the internet keeps evolving these problems will keep popping up. Hopefully Google will reverse its decision to hijack error pages, just as Verisign abandoned Sitefinder (well with a little pressure).