So I recently removed my last hard list (other than inbox and sent), in order to simplify things. Initially I was using the "Personal" hard list for everything. Today I tried to add something to one of my smartlists and it "vanished" from that smartlist because it didn't have a due-date associated with it when first created. I went to my inbox to look for it there but didn't see it. Luckily I have another smartlist that searches for untagged items and it was there. But I was confused as to why it wasn't showing up in my inbox. Then I saw that it was assigned to "List: Personal"! I went back to my settings and confirmed that this no longer existed. Apparently RTM was still using it as the "default list" despite it's non-existence. Not good! I'm actually surprised that we haven't seen more errors due to this, but I guess people don't change lists enough?

Anyway, obviously when a list is deleted that is set to default, the default marker should then be transferred to another list either automatically or by giving the user a choice, or simply by preventing the user from deleting the list until he's removed the default marker.