As arne suggested, it is your own code that is causing the conflict.
If you are using the AfterUpdate event of the form, you are causing an infinite loop. Luckily, newer versions of Access recover gracefully from this rather than freezing as earlier versions did. The AfterUpdate event of the form runs AFTER the record has been saved so if you modify the record in that event, you are forcing Access to have to save the record again and that causes the AfterUpdate to run again where you dirty the record forcing Access to save the record again. Get it?
If you are using the AfterUpdate event of a control, you will only catch null values when the user clears an existing value.. But the bottom line as namliam said is to not have a check field at all. There is absolutely no difference between checking one field for null or a different field for true. So, remove the check field entirely.