As the field of visualization continues to expand, understanding how visualization designers think and the kinds of assumptions they hold becomes crucial for creating and communicating clear, accurate, and impactful visualizations. In this exploratory study, we analyzed Twitter threads discussing visualization, aiming to uncover the beliefs and assumptions held by visualization designers. Utilizing 8 Twitter threads rich in discussion as case studies, we reveal the implicit assumptions that designers harbor while creating visualizations. If unaddressed, these assumptions often lead to confusion or misinterpretation. This paper discusses several strategies that designers can leverage to increase awareness about the assumptions they make. Though this study has limitations, given its reliance on inferred findings from self-reported Twitter data, it offers valuable insights for visualization design practitioners and proposes directions for future research. By addressing the issue of implicit assumptions by visualization designers, we can enhance the effectiveness and clarity of visualization communication.