You know what's worse than not being able to log in?
This is a submission for DEV's Summer Bug Smash: Smash Stories powered by Sentry . You Know What's Worse Than Not Being Able to Log In? Being told everything worked right up until you try to actually use your account. Yes, that was a real bug. And, somehow, I ended up being pulled into another authentication mystery. At this point, I’m starting to think authentication bugs have a personal grudge against me. 😅 In my previous Smash Story , I wrote about a bug where users simply couldn't log in. This time, the problem was sneakier because most of the flow looked completely healthy. The user was approved, the background task ran, the email and SMS arrived, and Cognito had a user. Then the user actually tried to use their account. And everything fell apart. It Started With Two User Pools The authentication setup was fairly large and had evolved over time, so there wasn't one shiny User Pool doing everything. We had an older Cognito User Pool supporting existing authentication flows, including mobile-based signup, while a newer User Pool handled a newer flow where users received an email containing their PIN. Both pools were intentional because they supported different parts of the authentication journey. That wasn't the problem. The interesting part was that the application database had its own representation of a user, while Cognito had another. On top of that, some of the work connecting those two systems happened asynchronously. As long as everyone agreed about who the user was, nobody cared. The moment they disagreed, authentication became very interested. The Tiny Timing Window The problem appeared in the partner and dependant journey. A member could create a partner or dependant during signup or later from the member details area. A relevant non-member user would then approve the account, which scheduled an asynchronous task called SendingEmailsAfterApprovalBot in a TaskList database table. That task ran every 15 minutes, and once it executed, the partner or depend