When an item is placed in the enchanting table, 1-2 of the enchantment options will not display their names when hovered over with the mouse, and can't be clicked on to enchant the item. The number of options displayed/able to be clicked will change after an item is enchanted, but at least 1 option is always inaccessable.
Update: This bug affects all enchantable items - books, all tools, swords, even fishing rods.
Update 2: This bug occurs on both SMP and SSP.
What item do you try to enchant?