Sorry about the super long delay! I had forgotten my password, and didn't have access to the source code repo where this bug was discovered. The "description of a bug" is a bit difficult to describe, so please bear with me:
I have a small jQuery plugin called jquery.datetime-picker.js, which overrides $.datepicker.formatDate() so that it can format a timestamp with time, in addition to the date. To do this, the plugin needs access to the _curInst variable. Everything works great while the datetime picker is open, but when it's closed, the _curInst variable is still available, so the plugin gets confused.
Here's a super-stripped down demo that includes some of the plugin code, plus shows the bug in action; normally it should append "12:00 AM" to the string returned by $.datepicker.formatDate() **only when a datepicker with
showTime param set to true**. The bug is pretty obvious here: the "12:00 AM" does not go away when the datepicker is closed.
Switch to jQuery 1.7.2 + jQuery-ui 1.8.18 (which contains this patch (!)) and the bug is fixed.
TL;DR: This bug breaks any jQuery plugin that needs access to $.datepicker._curInst.