[AccessD] A2003:Error message

Hi all
I am opening a form (frmRegistrations) from a Sub form on the clients form
(frmClients)
by double clicking a field in the sub form. The name of the field being
double clicked is (txtRegistrationID)
The name of the sub form is called sub_frmClients

Code goes something like this
In the double click of txtRegistrationID.

DoCmd.OpenForm "frmRegistration", acNormal, , , acFormEdit, acDialog
Forms!frmRegistration.RecordsetClone.FindFirst "[RegistrationID]=" &
Me.txtRegistrationID
Forms!frmRegistration.Bookmark =
Forms!frmRegistration.RecordsetClone.Bookmark

So far so good the relevant form opens and Access sends it to the correct
record
In relation to what was double clicked in txtRegistrationID.
But..
Sometimes - not all the time - if I make a change to any values in the
records
Of the newly opened form, I get the error message..
"Update or CancelUpdate without AddNew or Edit
OK or HELP are the buttons
When I push help it basically explains what the error message says.

So...
Can someone please tell me 
Firstly, Why I get the error at all?
Secondly, why I don't get it all the time?
Thirdly, what do I have to do to make it go away? :-))

I have been opening forms this way from other forms for ages 
In fact I copy and paste the relevant code from old dB's where
It is working fine

Any help MA

Many thanks

Darren