InfoPath 2007 - Fix for "Operation could not be completed" errors when opening InfoPath form in design view

If your InfoPath 2007 form suddenly stops working in design mode and you start to get the following error when opening the form in design mode:

"Operation could not be completed."

There are a few things you can do to try and fix it:

1. Remove all the the local cache files from your machine (typically found in C:\Users\USERNAME\AppData\Local\Microsoft\InfoPath) - delete the "Designer2", "Cache2" and temp folders
2. If that doesn't work, try running the form (e.g. in Visual Studio 2008, just right click on the form project and Click "Debug -> Start New Instance". When it opens, it will correct the project structure or re-add neccessary .NET assemblies to the project. Go to design mode again - and it will all start working (though all the tabs in the Designer window will now open). I suspect this happens if the xsf file is not correctly written to or some of your supporting files are readonly on save.