Create a new database:
Field A, numeric integer
Field B, numeric integer
Field C, numeric integer, formula A+B
Add a few records.
Select Field C and from the menu select “Hide Field” so now only fields A and B are visible.
Make a new record, type tab, try to enter something in field A and you can, but you can’t get out of the data entry field. Then if you try and quit, you can’t, have to force quit.