Question: We have had issues with Mootools not being very backward compatible specifically in the area of drag and drop functionality. I was wondering if anyone has had any similar problems with jQuery not being backward compatible. We are starting to use it quite heavily and are thinking about upgrading to a newer version to start using several plugins that require it. Will we have any issues if we get rid of the older version?

Answer: 
jQuery is so serious about backwards compatibility that they produce a "backwards compatibility" plugin for each release: <http://docs.jquery.com/Release:jQuery_1.2#jQuery_1.1_Compatibility_Plugin>. It let people who don't need backwards compatibility save on page weight.
*My experience* is that when upgrading, I sometimes find that stuff that used to work breaks. For instance, when upgrading from 1.7.2 to 1.8.3 I found some drag-and-drop features stopped working. Some problems may be due to deprecated jQuery functions being dropped, still in use by incompatible versions of [jQuery UI](http://jqueryui.com/).

Here is a [quote from another user](http://sympmarc.com/2011/06/27/jquery-and-backward-compatibility): 

> 
> Backward compatibility may not be the jQuery team’s strong suit, but
>  as with most things, it’s a work in progress. I’m not meaning to
>  disparage the great work they do, but it points out the fact that you
>  simply *must* always test your scripts with new versions of jQuery. 
> 
> 
>