Amazon Lex now supports fallback intent making it easier to handle unexpected user input. Until now, you could use error handling prompts to respond when the bot could not match input to an intent. Starting today, you can add a fallback intent to help your Amazon Lex bot recover gracefully in such situations. It can be used to manage the conversation flow, execute business logic, or handover the conversation to a human agent. The fallback intent can be designed to trigger an AWS Lambda function and provide responses. You can now exercise a more granular control over the conversation flow.
from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/10/amazon-lex-announces-support-for-fallback-intent/