[smabie]

>The number of times that persisted variables defined in long-deleted cells have masked bugs is more than I'd care to admit.

Definitely. It happens particularly often when you change a variable name (and modify its definition) and forget to update the name for parameters further down in the notebook. Suddenly, without any warning, you are using stale data for your analysis, which can really throw a wrench in things. It would nice if when you changed a cell it erased all the old definitions in that cell.

And I don't think I'm the only one who doesn't have trust in their notebook definitions: I've noticed a trend among pretty much anyone who uses them that after they finish their analysis, they restart the kernel and rerun the entire notebook from start to finish as they have little faith that the results in the notebook are actually derived from the cells currently in the notebook.