Here is what I found.

Create a table name junk with two fields firstfld and secondfld
create a form with a textbox and a button
In form Init

USE Junk
ACTI SCREEN
CLEAR
DISP MEMO
Thisform.text1.value = m.firstfld

Run the form

Observations:
The DISP MEMO shows the two memvars as expected and they are PRIV to Init.
Then the textbox gets the value assigned from FirstFld, which one would expect woulkd cause a Var not found error.

After form is released DISP MEMO show not trace of the memvars. Clicking the button again the DISP MEMO shows no sign of the vars (as expected) but there is no error on the assignment line and the value is properly assigned.

I could find no trace of explanation of this behavior in the help file.