If you have a Cpanel account, there is a section in there called "error pages" in which you can paste the HTML code of whatever page you wish to come up when a person receives that error, or types in that no longer valid URL (URL = a website address, whether it be the main page of a website or a webpage within that site).
For example, I have it set up so that if anyone types in a URL with my site as the domain, and the URL is not valid, normally they would go to MS's ugly white "404 Document not found" page. But I pasted the entire HTML code of my homepage into the 404 page in Cpanel, and now, no matter what anyone types as a URL for my domain, if it is not valid they will get my homepage. That way they can navigate to the correct page.
You can do the same for "400 Bad Request". Some people opt to have a customized 404 error page telling the visitor that they entered a URL that is not valid, but I just have them view my homepage so they can navigate to the correct page.
Secondly, a favicon is an image that appears right before the URL in the address bar. It also appears when you save it as a favorite in your "Favorites" (aka "Bookmarks"). It is not a big deal that it shows up in your "bad request" log. They still saw the page. Just ignore it.
Favicon = Favorite Icon
This file is found in the root directory of a website (so that each page can have use of it).
You can also make it appear before the URL in the address bar of your browser even if you have not saved it to your favorites by clicking on it and dragging it to the right staying within the address bar, and then dropping it within the address bar. Do this twice. Then try refreshing your page. If it doesn't show up, just close your browser, and open it up again and it should show up. Try it at google.com.
To see Google's Favicon, go here [xkalybr.com].