Recipe:

- start up conkeror
- do C-x C-f bla://asd.fg
  an alert box is shown telling us that bla is not a registered protocol 
- after clicking OK an exception is shown in the mini-buffer
- do M-n and from now on the tab-bar is no longer refreshed when switching buffers with M-n and M-p
  until tab-bar mode is disabled and re-enabled (twice M-x tab-bar-mode )
  instead each buffer change triggers an error
- last step reveals the tab for the buffer for bla://asd.fg.

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

same behaviour occurs with

download_buffer_automatic_open_target=OPEN_NEW_BUFFER;

Fixed, although a dialog box is still shown if you enter a valid-looking URL with an invalid protocol.