I've gotten to a point where I get a single frame in a debugger ... Here's a healthy halt run from a playground: healthy halt and here's the empty halt: empty halt When I open a debugger I only see the one frame .. this is not quite right ... The other thing that happens in this mode is that halts don't halt the ui process and I can end up with a screen full of halt if I put a halt in a loop or worse I end up with infinite errors, because the UI process is not halted (at least that's my current hypothesis) ... There is a UI process and I have saved and restarted the image at this point: process browser In a helathy image the UI process stack looks very similar, but I do notice that I've got an OSSubprocess child watcher in the healthy image and it is missing from the sick image: healthy process It appears that I create this problem when I terminate a debugger that was brought up in an ensure block ... once I've done this the image becomes completely unusable (for development and debugging) and I have to create a new image from scratch ... I'm hoping that someone can give me the secret formula for repairing things ... Is the UI process instance cached somewhere and when I nuke the process running ensure blocks the debugger's ensure block doesn't run or some such thing? I'll follow this route as I investigate ... any help would be appreciated as this makes debugging very time consuming Dale