Find-as-you-type boxes are usually built by observing changes in a text field, and querying the server via AJAX for search results or suggestions when the field has changed.
A common problem with this implementation is that there is no guarantee that AJAX responses are evaluated in the same order as the original requests. The effect for the user is that the search results are flashing back and forth while the user is typing the query, and when the user has stopped typing the last results don't always match the final query.
You won't notice this effect while developing on a local development server like Webrick, which processes requests one at a time. You will however notice the effect after deploying on a production server with multiple concurrent worker processes. Consider using Passenger in development instead.