Sidebar position

Hi, my sidebar modules are moving (sometimes) from it's position to the bottom of the article. I cannot find why does it happen in one articles and in others not. Do you know where might be the problem?
(btw. when I choose "Sidebar" position from music state group after saving it turns itself into "sidebar" from Custom position group. Is this normal?)

Here are examples:

Broken sidebar module
http://swiatloczula.nazwa.pl/new/index.php/tworcy-54401/andrzej-jakimowski-34133

Working well sidebar module
http://swiatloczula.nazwa.pl/new/index.php/tworcy-54401/agnieszka-holland-79489

In most cases this problem is caused by some unclosed tags in the article. Check them out carefully.

True.
This night I found out that it comes from extra </div> tag that entered to html code all the time.
Well seems that tab page breaker must be done with a lot of carefulness.

Problem solved. Thank you for the answer.

Its common mistake - in most cases visual editor should catch this, but sometimes it doesn't. If that kind of problems happens, its easiest to try to validate the site.