I discovered this behavior in Oracle APEX 4.2: you create a page with form elements and now when you press Enter in a textbox (text field), the page submits, even though you have "Submit when Enter pressed" set to No and haven't made any dynamic actions or other javascript checking for the keycode 13.
This can also end up making it look like you submitted twice: first with a blank/empty request, and then with the actual request. Interestingly, the first blank request is the only one Firefox sees, but Chrome and IE see both requests.