Does your company use Excel 2007 and 2003? Did you design a spreadsheet in 2007 with lots of pretty formatting, conditional formatting, IFERRORS, and the likes, only to have someone save the file in Excel 2003 effectively ruining your hard work! Here's the solution I've been using for that;

In the ThisWorkbook object, use the following code:

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

If Application.Version = "11.0" Then
MsgBox "You are using Excel 2003.  If this file is saved, functionality will be automatically deleted because of version incompatabilities.  Saving has been DISABLED.  THIS FILE IS NOT SAVED!!!!!!!!!!!.", vbCritical
Cancel = True
ElseIf Application.Version = "10.00" Then
MsgBox "You are using Excel 2002.  If this file is saved, functionality will be automatically deleted because of version incompatabilities.  Saving has been DISABLED.  THIS FILE IS NOT SAVED!!!!!!!!!!!.", vbCritical
Cancel = True
ElseIf Application.Version = "9.00" Then
MsgBox "You are using Excel 2000.  If this file is saved, functionality will be automatically deleted because of version incompatabilities.  Saving has been DISABLED.  THIS FILE IS NOT SAVED!!!!!!!!!!!.", vbCritical
Cancel = True
ElseIf Application.Version = "8.00" Then
MsgBox "You are using Excel 1997.  If this file is saved, functionality will be automatically deleted because of version incompatabilities.  Saving has been DISABLED.  THIS FILE IS NOT SAVED!!!!!!!!!!!.", vbCritical
Cancel = True
Exit Sub
End If
end sub
And users simply won't be able to save the file in an older version of Excel. Yay!

You may want to put a warning to that effect on the Workbook_open event though, so they don't do 5 hours of work and end up not being able to save it!