2. Not checking and understanding the sizing system used by the brand. Check the size table, it might be different from the one you know as there are many sizing system in the world. The most acceptable one is the EU sizing system, but it is still subject to change on whatsoever website you are ordering from.
3. Not putting the fabric type into consideration. Check the fabric type, especially if your body measurement is between 2 different sizes, if the fabric is a spandex fabric, which means that it is stretchy/knitted, you can go for the smallest of the 2 sizes. For woven fabrics, which are not stretchy, you can go for the other bigger size which can be perfectly slim fitted or tailored.