I personally don't like fixing stuff after somebody. Particularly in wordpress :P.

http://greenteam.com.pl/oferta/

Read this site. I added second row of buttons (it's not necessary to know concept of what :P). But, as you can tell, it will get completely damaged whenever you rollover among the second row options. I'm able to find anything wrong inscascading stylesheet and that i have no clue how this wordpress code works:

<?php if (is_tree(6)): //podstrony oferta ?>
<ul id="submenu" class="current_<?= $post->ID ?><?php if (is_page(6)) {echo " main";} ?>">
    <?php wp_list_pages('depth=1&sort_column=menu_order&child_of=6&title_li=') ?>
</ul>

Any clues? I'd appreciate any help. Could it be wordpress fault? Or perhaps is it inscascading stylesheet?

Should you check firebug, you can observe the height in the link <a> changes from 37px to 39px. Due to the floating property, everything changes right from the greater block.

I believe it is advisable to check all of your height qualities, since it is quite strange to alter the peak by 2 pixels on the hover event.

.oferta #content #submenu li a:hover {
    height: 39px;
}