My UX designer couldn't log in to my demo and I had no idea why

Last week I shared a new project with our UX designer. Sent her an ngrok link, asked her to try the onboarding flow. She clicked around for a few minutes, then messaged me: "I can't get past the login screen." I checked my code. Everything looked fine. I tried the flow locally — worked perfectly. I asked her to try again. Same result. It took me twenty minutes to figure out what happened. The frontend was loading through the tunnel, but the login form was still posting to localhost:3001 . The API call was silently failing on her machine because localhost doesn't exist there. On mine it worked because the API was running locally. Twenty minutes of debugging for something I would have seen in five seconds if I could have just watched her session. The paradox At work, we have error tracking, analytics funnels, session replay, performance monitoring, log aggregation. Dashboards for dashboards. All for apps that already work, where the decisions are incremental. But when we're building some