>>>I am trying to incorporate into my project a masked input plug-in (URL: http://digitalbush.com/projects/masked-input-plugin/)
>>>
>>>When I add this .js
>>>

>>><script src="Scripts/jquery-1.7.1.js" type="text/javascript"></script>
>>><script src="Scripts/jquery.maskedinput-1.2.2.min.js" type="text/javascript"></script>
>>>

>>>to a separate .aspx file in my project, the mask() function works very well.
>>>
>>>But when I add it to a page that uses Master page, I get error:
>>>

>>>0x800a01b6 - Microsoft JScript runtime error: Object doesn't support property or method 'mask'
>>>

>>>
>>>The test code is identical in separate test page and the page that uses master page:
>>>
>>>

>>> <script type="text/javascript">
>>>        $(document).ready(function () {
>>>             $("#txtPhoneNumber").mask("(999) 9999-9999");
>>>        })
>>>    </script>
>>>

>>>
>>>The error (above) happens on the line .mask() above.
>>>
>>>How would you recommend to debug this situation? TIA
>>
>>Do you have both scripts included into the master page ? (jQuery and plug-in)?
>
>Yes, I have both scripts in the master page. And I also tried to add these scripts on a page where I use them. But to no avail.

Make sure they are included in the right order and also try refreshing everything. I think I had similar problems when one of the scripts was not correctly loaded.