Another possibility to break tab-bar-mode:

- set download_buffer_automatic_open_target=OPEN_NEW_BUFFER_BACKGROUND;
- C-x C-f conkeror.org RET
- s 0 RET
- M-n
- C-x k RET

-> download buffer is killed, but its tab remains present. addtionally the minibuffer says: "call
interactively: Internal Error: too much recursion