Insufficient requirements and acceptance criteria.
Poor planning is one of the most common mistakes during Dynamics 365 solution implementation. Lack of precise requirements, scope definition, and resource allocation can easily increase the cost of projects and lead to new unexpected timelines.
Expectations and roles for each team member in the project.
Each project member has certain responsibilities or specific tasks that contribute to success. A common mistake is not to define roles and responsibilities during the planning phase, which can cost the project duration.
Priority on the must-have and nice-to-have functionalities for go-live.
Most clients demand some of the functionalities that are unnecessary for the go-live phase. The results of these actions often lead to poor and unfinished sales processes in the system, a lack of the proper instruments for analysis of the data, and end-user frustration.
Here are some standard functionalities that we are typically considered must-haves and nice-to-have in the process of the Dynamics 365 Sales implementation:

Must-have:
- Lead and Opportunity management
- Product Catalog
- Quote manager
- Integration with Outlook
- Views and forms implementation
- Data security
Nice-to-have:
- Sales Dashboard and Reports
- Social Selling
- Sales Collaboration
- Mobile App
Allowing unqualified users to perform quality assurance (QA) tests.
Permitting untrained individuals to conduct QA tests on a system can generate various adverse effects. Firstly, inaccurate results may arise due to their lack of understanding of the testing process and expected outcomes.
Secondly, the testing process may be delayed as unqualified users may not have the knowledge or skills to conduct effective and efficient testing.
Finally, incomplete testing coverage may occur as unqualified users may not be aware of all the critical functionalities of the system that need to be tested, leading to a lack of comprehensive testing coverage.
Therefore, it is crucial to have qualified and experienced individuals to conduct QA tests to ensure accurate, timely, and comprehensive testing coverage.
Deviations from the project timeline and changing requirements in proposed solutions.
Not adhering to the estimated project timeline and introducing new requirements that significantly alter the proposed solution can result in various issues. These include compromised solution quality due to the impact of changes on the final solution, an increased cost that can result from additional development and testing, and a heightened risk of project failure resulting from significant alterations to the proposed solution.
It is, therefore, essential to adhere to the initial project plan and carefully evaluate any proposed changes to minimize the risk of these issues.
(Bonus) Not learning out-of-the-box Dynamics 365 for Sales functionality before starting complex customizations.
Dynamics 365 for Sales has a lot of out-of-the-box functionality. Not knowing it can lead to rework with customization, which can add to the budget and complexity of the system. Also, it takes time to get used to functionality and platform. Investing in Dynamics 365 for Sales knowledge in the customer team can significantly increase requirements quality and reduce project budget and complexity. Therefore, it is highly recommended to do it before the project starts.