How a React useEffect Bug Caused Random Haptic Feedback and Tanked App Retention

The Problem: Random Vibrations Users Reported

A developer received multiple user emails over four months reporting that their app "vibrates randomly" or "keeps vibrating for no reason." Initially dismissed as user confusion, these reports turned out to be accurate bug reports. The issue was causing significant retention problems: 7-day retention dropped to 18% compared to a category benchmark of 35%.

Debugging Challenges

The bug was difficult to reproduce because haptic feedback feels different across devices. On flagship phones like Pixel, the motor is precise and subtle, while on mid-range devices like Moto G series and Redmi Note, the motor is stronger and more noticeable. The developer couldn't feel anything on their Pixel device during testing.

Debugging tools used:

• Reactotron to track re-renders and narrow down the problematic component
• BrowserStack for remote device testing (but haptic feedback doesn't transfer through live sessions)
• Drizz, which runs apps on actual physical devices and showed re-render frequency alongside haptic triggers firing in real time

The Root Cause

The issue was a React useEffect hook with a haptic call inside it. The dependency array had an object reference being recreated on every render instead of being memoized. This caused every single re-render to fire the haptic feedback on a screen that re-rendered constantly.

The fix: Wrapped the object in useMemo, which dropped re-renders and stopped the haptic loop.

Impact and Recovery

The bug had been silently affecting users for four months. After fixing the issue, 7-day retention recovered from 18% to 29% over the following six weeks.

Key Insight

Despite having Mixpanel, Amplitude, and custom event tracking set up, the most precise bug report came from plain text emails from users. The developer noted: "No stack trace. No device info. No steps to reproduce. Just 'your app vibrates randomly' and they were more right than anything my entire analytics stack told me in 4 months."