If you're using Microsoft Security to protect your database, some of your users will get a standard message box when they try to open a form they don't have authorization to access. This traps the error and provides a chance for you customize the message.
Insert this code into the on click procedure of the button that opens the form.
Private Sub cmdbutton_Click() On Error GoTo ErrorcmdButton
DoCmd.OpenForm "frmName", acNormal
ExitcmdButton: Exit Sub
' Traps permissions error message If Err = 2603 Then
MsgBox "You do not have access to this department's records" Resume ExitcmdButton