When I installed YOP Poll plugin a new menu item appeared in the sidebar admin menu. At the same time one of other items (for managing custom post type) just disappeared. Whenever I make changes to that menu item to make it visible another menu item disappears. Deactivating plugin makes all these menu items visible again. I need to mention I have about 23 menu items in this WP installation so the problem will arise only in such rare cases. I solved the problem by editing line 170 of inc/admin.php file. There’s an option of putting menu item into 29th position. I used decimal 28.5 instead of 29 as mentioned in WP Codex.