If you are editing a TFS process template you have the choice editing XML files or using the Process Template Editor within the TFS 2010 PowerTools. Unfortunately neither is fool proof. You can make errors than means the revised template does not fully work.
The worst of these errors will be picked up when upload the process template to a Team project Collection as during this process the XML is validated.
However, this will not find everything. Today after uploading a new process template I found I could not see two of my revised work item type in the list when I tried to create a new work item.
The best way I found to work out the problem was to try to import the .WIT file (from your local copy of the process template) again using either the Visual Studio –> Tools –> Process Editor –> Work item Types –> Import from File, or the command line tool WITIMPORT
In my case I then got the far more useful error
Error importing work item type definition:
TF237094: Field name 'Priority' is used by the field 'Microsoft.VSTS.Common.Priority', so it cannot be used by the field 'MyNewProcess.Priority'.
This gave me much more information to fix my problem