There are some instances wherein the customer needs to use special characters like the Yen sign (in those cases wherein they use a different currency). If the customer navigates to Setup > Customization > Advanced Printing Forms and customize any one of the standard templates there, special characters can be entered. However, some special characters like the Yen sign (¥) cannot be entered as is. For example, in some computers that has Japanese as the default language, the user has a way to enter the Yen sign on the keyboard. This will cause an error if the customer tries to save the form.
In order to avoid that error, instead of entering the special character as is, use the equivalent ALT code of that symbol. In the case of the Yen sign, type in on the keyboard this combination: Alt 0165 (hold the Alt key while typing in 0165).
For the complete list of Alt codes, please search "ALT codes" on your preferred search engine (ex: google.com).