emacs -Q
Manually resize the frame so that it is only a few lines high.
Do almost anything that would normally display another buffer, whether
in the same window or by splitting the window. For example, do C-h f
display buffer or do C-x C-b.
You get an error message with the subject line, and no buffer is
displayed. In fact, I tried to send a bug report with the same small
frame size, and the *mail* buffer got filled with (only) the error
message.
This is worse than useless, and represents a regression. If the window
cannot be split for some reason, then Emacs should do as it did
before: either use the same window or create a new frame.