Cryptic field and table names. Even more frustrating than duplicate names are cryptic names. There is no reason to limit the length of a field or table name, so use as descriptive a name as possible. Writing queries and debugging are much easier when the focus is the logic and not what T1C1x means. Missing or incorrect business rules.
Many businesses have strict rules that have nothing to do with program or database logic.
Do not neglect these rules. Missing or incorrect constraints. A very easy way to ensure that data are entered correctly is to use constraints. These can be implemented as checks to see if an entered value is within an approved list or range of choices. Constraints can also be implemented as masks that require phone numbers or ZIP codes to fit a specified format.