What mistake Did Anne make when Mr. and Mrs.Allan visit her place?

When the Allans are over, everything goes well until it comes time for Anne’s cake. Mrs. Allan declines to try it at first because she has eaten many other things, but she agrees to have a big slice when she finds out that Anne made it especially for her. When she takes a bite, it is clear that something is wrong with the cake, but she keeps eating it anyway. Marilla takes a bite and immediately asks Anne in shock what she put in the cake. It turns out that Anne added anodyne liniment because it was in a bottle labeled “Best Vanilla” (p.220). Anne couldn’t smell the difference due to her stuffed nose.