Image-based programming systems like Smalltalk tend to try to be totalising closed worlds, but inevitably there will be some need to interact with the outside. For example, code will be accessing a network connection, file handle, or database cursor: these references to the outside world won't be valid any more after storing and resuming the image.
Other languages deal with these sorts of references by making them unserialisable. That isn't an option for these systems (or is it?), because the entire "world" is serialised when execution stops, or even while it's happening. From the program's perspective, it's been running the whole time, even after a lengthy pause.
What happens when an image resumes execution while invalidated references are held in variables? Are they transparently nulled out, do they error when accessed, or does this situation just never arise in the first place by construction?