Most of us often play the roles of customers as well as vendors. When we are customers, we are likely to make the same mistakes that we see as mistakes of our customers while we serve them as vendors.
Guiding an expert
Hiring the experts and then guiding them about their craft is one of the most common mistakes. This does not only spoil the end result but also wastes the fees paid to the experts. It also kills the expert's sense of responsibility.
Not targeting the audience
When we are getting something made for our business, we must make it useful for the target audience. Our personal tastes and needs could be totally different. E.g. many film stars might not like to watch their own super hit movies.
Conflict of interest
Validation from inexperienced
We often tend to validate the work or ideas of an expert from a person with less or no experience in that domain. It is a natural human tendency to give suggestions. They want to prove their supremacy, especially when we put them in a higher position by asking to validate somebody else's work. Therefore these inexperienced persons usually end up giving their feedback full of disastrous suggestions in just a few minutes, to improve (?) the craft which took weeks or months for an expert to create.
Human brain is wired for survival. In-house experts or existing vendors see someone else's work as a threat to their own survival hence their brains naturally produce biased opinions.
Expecting too much in too less
Any product or service, especially a service, will always lose its quality if we always pay less and expect more. A quality product or service takes wholehearted efforts, quality time and skillful people - all of which has a high cost. If it is a onetime job then we must focus on quality. Cutting down one time cost generally increases recurring cost.
Not considering end to end solution is another mistake. A poor latter half wastes all the money spent on the first half. E.g. buying a quality product and using it inefficiently OR getting a good logo made form an expert and applying it incorrectly on the collaterals designed by inexperienced workers.
Wise customers and great leaders know the above, therefore they do all their research before hiring an expert. But once hired, they give the expert full freedom. Further if the expert's creations repeatedly fail to bring the desired end results, a wise customer simply finds another expert!