After upgrading to Internet Explorer version 10 (IE 10), attempting a search via Microsoft Search Server resulted in the script error: WebForm_DoCallback is undefined.
There is likely an issue with the server browser definition file. Some versions of .NET 4.0 seem unaware of IE10.
Hotfixes are available from Microsoft. The following page contains a description and pointers to the hotfixes:
Also, switching the Browser Mode setting to IE9 works around the error.
One customer resolved this issue by upgrading the server to .NET 4.5.