[Context and motivation] In addition to precisely representing requirements in specification documents and requirements management tools, practitioners use quick-and-dirty representations that are not compliant with academic guidelines or industry best practice recommendations. This includes drafted and informal representations in meeting notes, e-mails, and presentation slides. Sometimes, however, these representations may serve as a replacement for precise and officially specified requirements. [Question/problem] Although we presume that these requirements representations exist in most projects, the scientific community lacks evidence and characterization of these requirements. Studying these requirements is crucial because their use may pose severe challenges in later phases. [Principal ideas and results] We conduct an in-depth case study at a large-scale software and mechatronic systems provider. We use multi-method research, including a survey, interviews, and an analysis of processes and artifacts. We (1) explore instances and context of these requirements, (2) conceptualize the observations, and (3) define an ad-hoc requirement as one that is written, stored, or communicated in a suitable-for-the-moment manner. [Contribution] We position ad-hoc requirements (AhR) in the RE landscape and offer researchers a category of industry RE situations and challenges worth investigating.