For me, it was becuase I wasn't following the requirement for my lists to share 'as many nodes as possible with orig' in a particular circumstance.

This is issue is definitely related to the EmptyList at the end of the LL being changed. If you run into this issue make sure it isn't being modified anywhere. Don't just check your remove method. That test really needs to be updated to be more clear.

I am having the same exact issue. I am returning EmptyNode from the tail as tiagorg suggested. I used narve's test specs above with an additional one shown below to make sure it was the empty node from the tail.

Test.expect( list1.tail() === list1.remove(list1.head()) )

Any other ideas about why it might not be passing?

I'm still having this issue and it's definitely returning the same EmptyNode from the tail node.Can't figure out what the hell else it's expecting.

I was having this same thing. The issue is that I was always returning a new EmptyNode while this test is expecting it to be the same EmptyNode from the tail node.