Try moving the code to the BeforeUpdate for the Carbon Black field as follows:

[vba]

If Me.CarbonBlack_ >= Me.cboProduct.Column(1) And Me.CarbonBlack_ <= Me.cboProduct.Column(2) Then

MsgBox "CarbonBlack% is out of range. Valid range is " & Me.cboProduct.Column(1) & " to " & Me.cboProduct.Column(2)

Cancel = True

End If
[/vba]

Also, what is product col 1 and col 2 as the if statement doesn't look right?