Many-to-many Memes

The classic "just one small change" that breaks your entire data model. You design a perfect database with a unique constraint ensuring each user belongs to exactly one organization. The requirements were crystal clear. You even got it in writing. Then suddenly, the user who SWORE the relationship would "always be N:1" comes back asking if users can belong to multiple organizations. That look of horror is every database architect who now has to create a junction table, update all the queries, and pretend they're not dying inside. Next time, just assume every relationship is many-to-many from the start and save yourself the trauma.