The most significant backwards incompatibility was the introduction of .prop in 1.6.x which broke some uses of .attr ¹

I've been using jQuery extensively since 1.4 (albeit not as far as your 1.2) and don't recall any other changes that actually broke code.

¹ some people were using .attr to access some data which are strictly properties of a DOM object, whereas .attr is supposed to reflect the actual element as downloaded from the server. For example, .attr('checked') tells you whether a checkbox was initially checked, whereas .prop('checked') tells you whether it's currently checked.

The most significant backwards incompatibility was the introduction of .prop in 1.6.x which broke some uses of .attr ¹

I've been using jQuery extensively since 1.4 (albeit not as far as your 1.2) and don't recall any other changes that actually broke code.

There are also some features that are deprecated, but not necessarily removed yet (see http://api.jquery.com/category/deprecated/).

The jQuery team have also already announced plans for things they plan to deprecate in 1.8, and potentially remove altogether in 1.9 (e.g. $.browser)

¹ some people were using .attr to access some data which are strictly properties of a DOM object, whereas .attr is supposed to reflect the actual element as downloaded from the server. For example, .attr('checked') tells you whether a checkbox was initially checked, whereas .prop('checked') tells you whether it's currently checked.