Using acts_as_nested_set in ruby and something like taxon.move_to_child_of(parent) you can get this error if your table data is incomplete. You may have functioning nesting through the chain of parent IDs but you need the left and right fields as well. If either the left or right field is null you will get an error like this one for certain move operations. This can easily happen if you've created some dodgy fixtures for instance.
Trying using Taxon.rebuild! to repair the table.
Hello Ryan.. Hope you’re doing well… The feed doesn’t seem to be updating. (Nor is the side comment bar active) …
andrew / 12:03pm / 10 May 2010
Hopefully that fixed it. At least temporarily.
Ryan / 1:19pm / 10 May 2010
It might be fixed properly now. Although not the chat part. I should sort that out.
Ryan / 1:19pm / 10 May 2010
Chat is fixed now too.
Ryan / 1:29pm / 10 May 2010
Helped me a lot 2 years later and using another gem awsomenestedset
Rogério / 4:28am / 9 November 2012
Thank you!
Sebastian / 5:36pm / 29 August 2013