Saving Post redirects to 404 page when php $_GET variable in post body

I am having a very annoying issue where certain posts redirect to 404 page when trying to save them. The posts do not save. It only happens to a few posts and did not happen prior to the last few WP updates. I tired disabling all plugins and using different themes with no change. I also tried creating a new post on a different WordPress installation again with no change.

In one post I narrowed it down to one line that if taken out solves the issue, though there is no such line in another post that gives 404 error.

$_GET["ip"];

I realise this is a PHP global variable but I don’t see why it should cause an issue in a post. The PHP variable is part of the code I am displaying within pre tags. Another post that redirects to 404 save just fine when it was first written and so did the post with the GET variable.

Can anyone explain why this is happening? It seems like a bug to me.