The first mistake is treating brand reputation as a shortcut for suitability. La Roche-Posay and Aveeno both cover multiple product types, so one excellent fit does not mean every product from the brand will suit your skin or routine.
The second mistake is ignoring texture. Rich balms, light lotions, gel cleansers and creamy cleansers all feel different in daily use. If you dislike the finish, you are less likely to use the product consistently, no matter how good the offer looks.
The third mistake is comparing a specialist face product with a family-size body lotion. They may both sit in Skincare, but they are not interchangeable. Decide whether you are buying for face, body, targeted care or general moisturising before comparing prices.
The fourth mistake is buying the largest pack before you know you like it. Larger formats can offer better value, but only when the formula suits you and the product will be used regularly.