If you have users complaining about errors when editing or adding work items from Excel -this might be one way of help you capture what they were doing -and the cause. 

For instance the error below was generated when attempting to publish a workitem with text for a work item type that didn't exist.

[Info, PID 5664, TID 6720, 21:20:21.410] Managed ELead Excel AddIn loaded
[Error, PID 5664, TID 6720, 21:24:46.748] {

[Error, PID 5664, TID 6720, 21:24:47.275] Exception:  {
Exception Message: Input string was not in a correct format. (type FormatException)

To get enable tracing please create a new configuration file where Excel.exe exists called Excel.exe.config and place the following XML in that file:

<configuration>
   <system.diagnostics>
     <switches>
        <add name="General" value="3" />
     </switches>
      <trace autoflush="false" indentsize="3" />
   </system.diagnostics>
</configuration>

If you run the TFS add-in and a problem occurs, an error log called tfs_msexcel.log will be generated. 

(Actually the log will be created even if you don't have an error -just nothing gets appended to it)

 

Oh yeah this works the same for project too!