The hidden danger behind this technique is only revealed if the user moves very quickly through the input fields, or the server connection is slow; in these cases, the web application would attempt to make new server requests through an XMLHttpRequest object that is still busy waiting for the response to a previous request (this would generate an error and the application would stop functioning properly).
Re: Drawbacks of Thread-Safe AJAX
The hidden danger behind this technique is only revealed if the user moves very quickly through the input fields, or the server connection is slow; in these cases, the web application would attempt to make new server requests through an XMLHttpRequest object that is still busy waiting for the response to a previous request (this would generate an error and the application would stop functioning properly).