Bug? Strange between Wizard control and client javascript code

[Guest]

I have two textboxes (tbox1, tbox2) in a step of Wizard control. I use
javascript code to clear tbox2 when onblur occurring on tbox1:

tbox1.Attributes["onblur"] = "javascript:var tbox2 =
document.getElementById('<%=tbox2.ClientID%>'); tbox2.value='';";

1) I tried to put some non-empty string data in tbox2 and press next button.

2) Then I go back to previous step and leave focus of tbox1 and the screen
showed tbox2 is cleared by the javascript code.

3) I click next button again However, the code in Wizard1_NextButtonClick
still get the value of tbox2's previous non-empty string value.

However, I did create a simple test page and the step (3) got the expected
empty string.

Anything setting I missed? Or the wizard control's bug?