When a turtle update operation cannot finish because the underlying canvas has been cleared (when the STOP button is pushed), turtle.Terminator is raised. When turtledemo runs the main function of a demo, it catches any Termininator raised before main returns.

However, special demos leave an event loop running after main returns. If the demo is free running, as with clock and minimal_hanoi, clicking STOP causes such exceptions. (This is not be a problem with paint as updates only happen in response to mouse events on the canvas and complete before a user could move the mouse to the STOP button.)

This is the clock trackback, with common file prefix removed:
\tkinter\init.py", line 1487, in __call__
return self.func(*args)
\tkinter\init.py", line 532, in callit
func(*args)
\turtledemo\clock.py", line 116, in tick
second_hand.setheading(6*sekunde)
\turtle.py", line 1935, in setheading
self._rotate(angle)
\turtle.py", line 3277, in _rotate
self._update()
\turtle.py", line 2659, in _update
self._update_data()
\turtle.py", line 2645, in _update_data
self.screen._incrementudc()
\turtle.py", line 1291, in _incrementudc
raise Terminator

The hanoi traceback starts differently:
\tkinter\init.py", line 1487, in __call__
return self.func(*args)
\turtle.py", line 686, in eventfun
fun()
\turtledemo\minimal_hanoi.py", line 53, in play
hanoi(6, t1, t2, t3)
...<5 recursive calls deleted>
\turtledemo\minimal_hanoi.py", line 36, in push
to_.push(from_.pop())
\turtledemo\minimal_hanoi.py", line 36, in push
d.setx(self.x)
\turtle.py", line 1807, in setx
self._goto(Vec2D(x, self._position[1]))
\turtle.py", line 3178, in _goto
<last 3 lines as above>

These exceptions and tracebacks do not stop the master demo window, but are printed to the console (python -m turtledemo) or Idle Shell (open in editor, run). They are ugly, might unnecessarily alarm a naive user, or falsely teach that tracebacks are to be ignored.

In the patch to clock.tick, I put try: at the top, to be safe, although just before the update of the second hand might be good enough.

New changeset a43d03cdf38b by Terry Jan Reedy in branch '2.7':
Issue bpo-21823: Catch turtle.Terminator exceptions in turtledemo.
http://hg.python.org/cpython/rev/a43d03cdf38b

New changeset 1ae2382417dc by Terry Jan Reedy in branch '3.4':
Issue bpo-21823: Catch turtle.Terminator exceptions in turtledemo.
http://hg.python.org/cpython/rev/1ae2382417dc

In 2.7, the exception in tick occurred at tracer(False), so I moved try: up in all patches.

Since hanoi do not have user interaction, once started, it does not need to be 'special'. Like planets_and_moon, it could run until done and then return 'Done'.