Debugging "Through" is really really awesome, except when it causes
your image to lock.

At first I thought it was because I had some DNU ahppening when I
pressed Through, like on the send to #critical:, here:

   |mutex| mutex := Mutex new.
   mutex critical: [ [Object new] do: [:err | ] on: Error ]

but then even after correcting it, it happened again:

    |mutex | mutex := Mutex new.
    mutex critical: [ [Object new ] on: Notification do: [ : noti | ] ]

I love Through, but I have no idea how to fix this..