One of my developers and I ran into an interesting problem today - we'd migrated a web site over to SharePoint 2007 (using a popular content migration tool) and found that all page postback calls in SharePoint were giving JavaScript errors, specifically:
Object doesn't support property or method.
Turns out that our master page and page layouts contained input controls (text boxes) with name/id as "submit". Use of this reserved name sent ASP.NET's JavaScript code into a tailspin and wrecked havoc with any postback submissions.
So, when you develop web sites in ASP.NET and/or SharePoint, be inventive with names for your buttons, text boxes and hidden fields - how about btnSubmit ;)
I read somewhere else that naming the main ASP.NET form "default" is a no-no also.