What environments are these deployed to (e.g. Development Web Server and IIS on Windows Server 2008)? What version of ASP.NET are you using (1.1 or 2.0/3.x)? What browser are you using (Internet Explorer, Firefox or Safari)?
It's possible that the servers have different configurations for their Browser Capabilities - which is why ASP.NET is rendering out different behaviour from each.
Depending on the version of ASP.NET there are different areas to check.
If you're running ASP.NET 1.1, then you'll need to check the machine and various web .config files found in:
C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322\CONFIG
For differences in the <browserCaps>
section.
If you're running ASP.NET 2.0, then you can check the machine and web .config files, but the <browserCaps>
section has been deprecated in favour of the .browser files that can be found:
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG\Browsers
Obviously, these can also all be defined in the local application .config files, or in the /app_browsers/ folder in the root of the site - but if they are exact copies on both servers, then this shouldn't be the issue.