I just spotted the "when tabbing out of a field" bit. I had it in my head this was all happening on save.
In which case, yes, I would consider moving the field to xf_forum (which is most correct anyway), and then the validate field method in the forum datawriter won't give this error.
Just to explain it in some detail:
The forum_edit form contains this:
PHP:
<form action="admin.php?forums/save" class="xenForm formOverlay AutoValidator" data-fieldvalidatorurl="admin.php?forums/validate-field" data-redirect="on" method="post">
Notice the field validator URL. This is the code that Lawrence is referring to.
At the moment, whenever a field is edited, that method is called to verify the contents of the form so far. This is the kind of thing that notifies users that their chosen username is already taken on registration without submitting the entire form.
Because your field is in the Node DW, the validator sees a value for a field named "newthread_button_title" and because it only checks against the Forum DW it fails and gives the error.
Lawrence's suggestion proposes modifying that validator - I don't personally think this is a great idea.
Lawrence's other suggestion (and mine) is to move the field to the Forum DW. IMO this is most correct. Think about the nodes we have by default. The only other one is Categories. You can't create threads in a Category so having the field available to categories doesn't seem logical. If some add-on came along and implemented something like "Article" nodes, again, there's no need for your field to be there, either.
However: there is one more solution:
In the field you have added to the forum_edit template, give it a class of "OptOut". That will actually prevent the error in its entirety without having to change much within the add-on itself.