Where it goes off the rails is when we embed a user interface inside another user interface. That's not a problem specific to the tabulator, but also with all novigation functions, like home, forward, and back.
I rarely use web-based email interfaces, but when I do, I accidentally send half-written email messages, because I'm trying to use a block quote or something similarly indented. It's especially bad when the next field is the 'Send' button, and pressing the spacebar sends the message.
Later, some systems, notably IBM's, overloaded this to move to the ‘next’ field on a form, which might be to the left and down the page. So from there MS-DOS/Windows stuck us with a Tab that sometimes moves horizontally within a text field, and sometimes moves to a different field. Just like Return sometimes moves to a new line/paragraph and sometimes submits a form.