What are the consequences of not disclosing all Conflicts of Interests (COI)? This is in follow-up with this question:

Clear explanation of Conflict of Interest with examples in journal articles

Apart from what it really is, it keeps me thinking why should authors disclose possible conflict of interests (for the publications that require them)? 
To be clear, what could happen if they don't? Is it anything to do with unethical conduct or does stating them support the authors in any way?
According to the Committee on Publication Ethics, retraction is a possibility.  See their Retraction Guidelines, page 2:


  Retractions are also used to alert readers to [...] failure to disclose a major competing interest likely to influence
  interpretations or recommendations.


The Retraction Watch blog has questioned how often this is carried out in practice.