App Interface Guideline: Feedback — Error Feedback. Keywords: inline error banner. Platform: iOS/Android/React Native. Description: Show clear error messages near the problem. Do: input-level error + summary banner. Don't: Only change border color with no explanation. Good Example: <TextInput ... /><Text style={{color:'red'}}>{error}</Text>. Bad Example: <TextInput style={{borderColor:'red'}} />. Severity: High.