I've searched all around for this one, so forgive me if I missed it.
In every theme I have tried, adding the event calendar causes the sidebar to appear on the very bottom of the page in IE. I don't use IE, but a number of visitors to the website do.
I've validated the XHTML
All I did was add the following code to the sidebar:
<?php ec3_get_calendar(); ?>
<?php ec3_get_events(5); ?>
It breaks the Freshy theme, Vermillion Christmas, and everything else except the Default theme.