The Events Calendar Pro (from http://tri.be/) has a few problems.
If you are trying to figure out why a calendar with no events in that month has completely screwed up header navigation, just put this line of code inside of table.php in the top of the display_day function (put it directly before the for loop):
$thisisherebecausethispluginSUCKSdontremoveit = has_excerpt() ? TribeEvents::truncate($post->post_excerpt) : TribeEvents::truncate(get_the_content(), 30);
This basically just runs some "truncate" function on the "TribeEvents" object. For some reason this truncate function magically fixes the header nav display issues.