A very small minority of sites encounter javascript problems in MSIE when editing events. You may get a message that reads something like this:
Message: HTML Parsing Error:
Unable to modify the parent container element before the
child element is closed (KB927917)
This is caused by the joomla javascript library (mootools) triggering an event too early.
Possible solutions:
1. Enable the mootools upgrade system plugin in Joomla 1.5.20 +, or
2. Install and enable this plugin (setting the config option to enable it to activate on event editing pages).