www.bbc.co.uk/news/uk-42036462
This will hit weekly paid workers - as December is a 5 week month, those who are paid weekly will be considered to be earning more in December despite the fact their weekly earnings have not changed.
Those affected by this to the point of UC being stopped rather than 'just' reduced will have to reapply (and then wait) for UC in the new year.
There was a previous thread discussing this particular flaw of the many flaws in UC here - www.mumsnet.com/Talk/am_i_being_unreasonable/3066503-Universal-Credit-wont-be-paid-in-months-with-5-weeks-WTF-AIBU-to-think-no-one-realises
And a petition here - petition.parliament.uk/petitions/203492