A developer's oversight in a chat application prototype led to unexpectedly high charges due to PubNub's MAU pricing plan, which counted each randomly generated user identifier as a unique user. This issue arose because the developer did not set custom UUIDs, resulting in inflated user numbers. In response, PubNub implemented a policy requiring developers to assign unique UUIDs to prevent such billing discrepancies. By enforcing this practice, developers can ensure accurate user identification, optimize the user experience, and effectively manage application data. To assist developers, PubNub suggests using built-in ID generators in programming languages or hashing unique device or user information to create identifiers, and provides resources like an online UUID generator tool and a Python script for this purpose.