Yes, I was trying to change Pledge from the email subject line on an invoice.  No, the error message was "you must enter pledge on the subject line".  I think as a work around, you are able to replace Pledge with Invoice but you must keep Pledge No. in the brackets

original entry on email subject line under custom style template:

Pledge[Pledge No.]

new entry 

Invoice[Pledge No.]

Now, the invoice will say Invoice 98 in the email subject line