var token = $('new_post').authenticity_token.value;
var post = new Request({url:this.getParent().getParent().getElementsByTagName('form')[0].action}).send('authenticity_token='+token+'&post[content]='+this.value);

Rails generates the authenticity token using the SecureRandom.base64 method provided by ActiveSupport to generate a random string. Unfortunately Base64 strings cannot be used directly in URLs because they contain ‘+’, ‘/’ and ‘=’ characters and it was this that was causing the bug: if the authenticity token contained a ‘+’ then it was being interpreted as an URL encoded space causing the exception to occur. Of course JavaScript provides a nice solution to this problem: the encodeURIComponent method.

You may also have noticed there’s another, potentially nasty, related bug lurking in that seemingly innocent bit of JavaScript: the post[content] parameter isn’t being encoded either. This means that if the user was to enter something like you really should &handle this=properly then the ampersand would be treated as a new parameter rather than part of the content string. Again encodeURIComponent comes to the rescue:

var token = $('new_post').authenticity_token.value;
var post = new Request({url:this.getParent().getParent().getElementsByTagName('form')[0].action}).send('authenticity_token='+encodeURIComponent(token)+'&post[content]='+encodeURIComponent(this.value));