If you start seeing this error (both when trying to order) and as emails to the sales address then the error is most likely that someone has deleted the template with id x but that is still set to be used in the database.
The solution for this is to update “System-Configuration-Sales Emails” in admin and save existing templates instead of the template that is now removed.
Note: Also it might be good to consider why this happened, is there some (real) problem or was the template file(s) updated and some old database template was removed [perhaps there is an educational need].