Posts falling below sidebar

with validation errors of this magnitude, the browser display is unpredictable.

also, your site seems to be missing a nesting structure to keep the divs contained.
as an example: #header is in #container; #left-column is in #container; #right-column is not;
these divs are probably floated – if the browser does not find enough space, it will drop the div; etc..

it’s back to basics – html and css:
http://www.w3schools.com/html/default.asp
http://www.w3schools.com/css/default.asp