not quite a Janus word, but almost...

Make the word 'boring' exciting by simply removing the 'r'.

— Moose Allain (@MooseAllain) 25 April 2017

The best of times, the worst of times

(Credit: http://www.walthandelsman.com/)

improving the negative

This came through in the abstract of a linguistics (AmE) dissertation/(BrE) thesis:

The ungrammaticality can be improved by supplying the expression with a tense or a syntactic context.

I'm pretty sure that means that the expression seems more grammatical if it has supporting context, but 'improving ungrammaticality' seems a different thing. My impression is that this kind of

negative thing + positive growth = less negative

interpretation is more likely to happen in passive contexts ('ungrammaticality was improved' versus the active 'this improves ungrammaticality').

Language Log covers a lot of similar things with regularity.