The latest iteration of OpenAPI Specification, OAS 3, has been adopted by many developers, but common mistakes are still being made when migrating from OAS 2.0 (Swagger 2.0). Using the old `swagger` property to specify the version is invalid in OAS 3, and instead, the new `openapi` property should be used with semantic versioning. Defining body parameters in the "parameters" section is also incorrect, as they should now be placed under the `requestBody`. Additionally, declaring response headers as a list of Header objects or an array of reference header objects is not allowed according to the specification. Furthermore, referencing undefined OAS 3 components without defining them first can lead to issues with tools that use this format. These mistakes can be avoided by using the correct syntax and adhering to the official specifications.