Hidden and Invisible Objects
By Diego Oppenheimer, 30 Jan 2009
When gathering data in Excel it is common to start with information found in a variety of sources, frequently lists or tables located on the Internet. Customers often copy and paste this data into Excel from the original source, but unfortunately this can also unintentionally paste many additional objects into the spreadsheet besides the data. These objects (shapes, text boxes, controls, etc.) are often not noticeable to the user after the paste, but can result in
slower performance and larger file sizes. One step customers can take to avoid unwanted, hidden, and invisible objects in their document is to find and remove the objects after the paste operation as part of their data cleansing process.
Complete article here:
http://blogs.office.com/b/microsoft-excel/archive/2009/01/30/hidden-and-invisible-objects.aspx