What mistake did Anne make with the cake?

Anne's mistake was her failure to notice the vanilla bottle she pulled out was mismarked, and that the bottle had been used for something other than vanilla....

"Mercy on us, Anne, you've flavored that cake with ANODYNE LINIMENT. I broke the liniment bottle last week and poured what was left into an old empty vanilla bottle. I suppose it's partly my fault--I should have warned you--but for pity's sake why couldn't you have smelled it?"