It may be legitimate in certain pre-calculated fields.
If some of your queries are expensive and you decide to go with pre-calculated fields updated automatically using database triggers, then it may be legitimate to keep the lists inside a column.
For example, in the UI you want to show this list using grid view, where each row can open full details (with complete lists) after double-clicking:
REGISTERED USER LIST +------------------+----------------------------------------------------+ |Name |Top 3 most visited tags | +==================+====================================================+ |Peter |Design, Fitness, Gifts | +------------------+----------------------------------------------------+ |Lucy |Fashion, Gifts, Lifestyle | +------------------+----------------------------------------------------+ You are keeping the second column updated by trigger when client visits new article or by scheduled task.
For such cases, keeping lists is legitimate. You just need to consider case of possibly exceeding maximum field length.
Also, if you are using Microsoft Access, offered multivalued fields are another special use case. They handle your lists in a field automatically.
But you can always fall back to standard normalized form shown in other answers.