<p>I have an asp.net image button and I want to cancel the click event incase he fails the client side validation... how do I do that?</p>

There is an OnClientClick event you can set this to your javascript function. If you return true it will continue to the post back.  If you return false the post back will not happen.
```
```
<asp:Button ID="NavigateAway" runat="server" OnClientClick="javascript:return PromptToNavigateOff();" OnClick="NavigateAwayButton_Click" Text="Go Away" />

  <script type="text/javascript">
    function PromptToNavigateOff()
    {
        return confirm("Are you sure you want to continue  and loose all your changes?");
    }

  </script>
```
```