You don't. Worst case is you had wrong assumptions and restart the program, same as if you didn't have the ability to change things.

Best case thought, it's so useful. It's also incredible for editing tunables on the fly(think videogame gameplay values, layout values for UI, etc).

In Smalltalk, most of the class library, and hopefully your own codebase, is either side-effect free or at least well encapsulated. The classic trip-up in Smalltalk was with Streams. An experienced Smalltalker would make note of where the code was fiddling with Streams, then go down the stack until the debugger has effectively gone back in time before the Stream objects were instantiated. Try that for a few seconds, then restart the app. If your environment was set up intelligently, none of that would take that long.

I believe changing data structures gives a "you must restart to make this change" when resuming debugging. It's mainly for code flow type bugs rather than data structure type bugs.

If the data structures in memory don't fit the new code, I suspect that's a compiler bug.