The copied & pasted form code injects a JS file which overwrites jQuery.ajax() in order to do some custom handling. This JS file is hosted by MailChimp.

Since jQuery 1.5, jQuery.ajax() has accepted two parameters, `url` and `settings`. The jQuery v1.0 through v1.4.4 API only accepted one parameter, `settings`. The MailChimp version of jQuery.ajax() also only accepts one parameter, such that any other code in the page that tries to pass two parameters will malfunction.

My tiny company is built around a WordPress plugin. It runs in cooperation with JS code written by hundreds of other developers on a daily basis. Sometimes conflicts arise. I debug them, and either fix a bug that I wrote and release a new version, or work with the 3rd-party developer to fix a bug that they wrote.

I've had several customers complain to me that my plugin doesn't work. I debug their site, find that jQuery.ajax() has been overwritten, trace it back to the MailChimp file, explain that MailChimp has made a mistake, this kind of thing happens all the time, and I will work with them to resolve the issue.

I've contacted MailChimp several times about this issue over the past 4 weeks. In their most recent response, they explained: "That being said, keep in mind that we serve over 3.5 million customers and even the slightest change can have a massive impact if not implemented carefully." Apparently they aren't concerned about the 3.5 million customers that are potentially being negatively impacted because of careless implementation in the first place (or at least careless maintenance).

Suspect MailChimp JS file: https://gist.github.com/colllin/6717655#file-jquery-form-n-validate-js-L1683

jQuery Docs: http://api.jquery.com/jQuery.ajax/