A foreign key on a self-referring table

In a project I'm currently working on, I looked at the database design chart (made by other people) and found that some tables have foreign keys referring to themselves.

For example:

Table Customers:
|--------------------------|
|CustomerID |(PK)
|--------------------------|
|Name |
|City |
|ZipCode |
|CustomerID |(FK)

(That's an example, of course, it's not my real case).

That's quite weird, in my opinion, but I wish to know whether it is acceptable, in general, or they are just violating some database design principle.

...Thanks!...

-- 
Andrea Laforgia