The three common CPQ design mistakes to avoid while implementing CPQ for your customer are:

A. Using price book entries to handle attribute based variationsinstead of lookup price rules: Price book entries are not designed to handle attribute-based variations. Instead, lookup price rules should be used. These rules allow you to adjust pricing based on specific product attributes, providing more flexibility and accuracy1.

C. Creating process builders and flows to pass data between fields instead of using twin field mapping: Process builders and flows can be complex and difficult to maintain. Instead, twin field mapping should be used. This feature allows you to map fields between objects, ensuring data consistency and reducing the need for custom code1.

D. Writing customizations for product selection or validation instead of using option constraints, product rules, and bundles: Customizations can be time-consuming and costly to implement and maintain. Instead, Salesforce CPQ provides built-in features like option constraints, product rules, and bundles that can handle product selection and validation. These features are designed to enforce business rules and ensure that only valid product combinations are selected1.