Tonight I copied an ASP 2.0 web site onto a client's machine, and lo! It did not work. It works perfectly on mine, under built in debugger and under IIS 7, and two weeks ago, it worked perfectly on his machine under IIS 6.
The only changes I have made are upgrading telerik RadGrid etc. to Q1 2009, and some refactoring. The application fails on two entry point web forms. Both have a RadGrid, which uses a User Control for its editing pop-up. Both forms fail to compile, with a 'Type or namespace JobEditControl not found" error, on a line like this:
var editControl = e.Item.FindControl(GridEditFormItem.EditFormUserControlID) as JobEditControl;
The control is not referenced in the markup, as it never appears on there, and as I understand it, it should be compiled into the same default namespace as the page is compiled into when first hot.
The machine is still serving user controls properly, i.e. the .ascx handler appears healthy, and a quick test page containing a quick test control I added worked fine.
Another developer was busy on the machine before me, also installing an ASP.NET application, but I can't think of anything he would have changed. Even being my competition...