I had 30 levels of xp, and a diamond pickaxe to be enchanted. I placed it in my enchanting table, the list of levels and enchantments did not appear, but since I forgot the lapis lazuli anyway I removed the pickaxe from the table and went to get the lapis. THEN I noticed in my inventory the pickaxe got somehow enchanted with Efficiency IV, my levels remained unchanged, and I never even used lapis lazuli with it.