It works now on the dev branch so you can close this report as fixed (even if I don't know which changeset fixed it).

Thanks for checking, Pantxo. I actually also only observe the segfault on Octave running in a VM with a single core so this might relate to other specific issues that already transpired in other bug reports; let's ignore this here.

If you add a line in the clbk function to display some text, you will notice that the callback is triggered as expected when pressing the button so the issue seems to be with the refresh of the figure and not the callback per se (adding a call to drawnow() in clbk fixes the issue, but you shouldn't have to).

Run the following and press the OK button - the plot will be updated immediately:

function testwaitfor
F = figure;
H = plot(rand(1,10),'Tag','plot');
h = uicontrol(F,...
  'Style','Pushbutton',...
  'String','OK',...
  'Callback',@clbk);
%waitfor(F);

function clbk(varargin)
H = findobj(gcbf,'Tag','plot');
set(H,'ydata',rand(1,10))

Now uncomment the waitfor(F) line: if you press the OK button, the figure will only be updated if you click on it or gives the focus to another window. I also observe that using Ctrl-C while Octave is in the waitfor results in a segfault.