When using TFS within Visual Studio, if you undo pending changes on an ASPX file by right-clicking on that file in Solution Explorer and selecting "Undo Pending Changes", pay close attention to the dialog box about what files are going to have their changes undone. TFS will not only undo changes on the ASPX file you selected, but will also undo changes for the code-beside file. I have lost my work a couple of times with this feature. Unfortunately, I don't know of a way to change that behavior. If you do, let me know!