Clipboard pasting only once?

The Office Clipboard enables you to copy multiple items
from various worksheets or other Microsoft Office
programs. You can then paste the items to the same
worksheet, to other worksheets, or to other Office files.

The cut or copied items are collected on the Clipboard
toolbar which automatically appears as soon as two items
are cut or copied in a row. You can continue to add to the
Clipboard even after the toolbar appears. The Clipboard
toolbar also includes a Copy button you can use to copy
items.

The Clipboard toolbar displays one icon for each of the
cut or copied items. When you point to an item on the
toolbar, its contents are identified in a ScreenTip. You
can paste one item at a time or use the Paste All button
to paste all the items at once.

Although you can continue pasting text or numbers to
different locations using the Clipboard toolbar, there is
one exception to this function that can only be pasted
once. What item must be pasted immediately and can only be
pasted once?