The task killer are programs that can force the closure of processes and applications on your Android smartphone. But not all applications respond well to forced closure. WhatsApp, for example, could temporarily disable (or, in Android 2.1 or earlier, even permanently disabled).
The problem is that WhatsApp servers are not able to register this type of seating, and the result is that the message delivery delays can be performed even several minutes.
In this case you do not receive an error message, but WhatsApp will malfunction for a while. All you can do, once everything's back to normal, is never close the application with a task killer.