Allergen cross-contact is the unintended transfer of an allergenic ingredient or its proteins into another food. Shared knives, fryers, preparation surfaces and serving utensils can be routes of transfer, even when the unwanted ingredient is not visible.
It differs from microbial cross-contamination. Ordinary cooking does not reliably eliminate the allergen. Ingredient checks, separation and suitable cleaning are necessary; removing a visible garnish is not equivalent to preparing an allergen-free dish.
Reference: Further reading.