Designers often have a strong desire to solve problems for their clients. However, instead of consulting experts, some attempt to perform the tasks themselves. You can find contractors and installers playing designer or interior designers playing architect.
A well-known kitchen designer claimed to have knowledge of architecture. She only measured and put on plans the portions of the building that was to be worked on. She would refer me for producing drawings to obtain building permits. Every single time, I found architectural issues in her design due to the adjacent areas not being considered. Once she was moving a door over the existing under-floor access. Another time I found her new window conflicted with the pipes on the exterior wall. The worst was when she was removing 20 feet of a wall that supports the exterior wall of the upper level. In addition, clients were often not thrilled to find out they had to pay additional for drawings required for building permits. Especially after they had paid a significant amount towards her kitchen design. This designer supposedly charges a high design fee.
When I was a Kitchen and Bath Designer at a retail store, we often had customers bringing in architectural plans for a cabinet quote. A good portion of these plans of new homes had ill-planned kitchens. It was tricky to tell the customers the designs need to be reworked without throwing their designers/architects under the bus. Kitchen and bath design is not difficult, but there are a lot of details to pay attention to.
The most common mistake, in my opinion, is hiring someone who can work drafting programs to design and create plans for construction projects. One does not need to be an architect to design homes. However, it is necessary to have architectural education and training, drafting skills and experience, and understanding of building codes and regulations.
Building projects today can be complex and require multiple experts. A good designer should act as a project coordinator and inform clients at the beginning of the projects what consultants are needed. Clients should understand nobody can know everything. Any professional is happy to provide proof of their expertise and answer questions.