What can happen to a paper submitted without co-author knowledge
Usually, nothing happens, except damage to the submitting author's reputation.

Anything could happen.  It depends on the coauthor's feeling and the content of the paper.  If the content of the paper is objectionable, then this might lead to retraction.  Potentially the submitting author could be fired.  But most likely, if a coauthor complains, the submitting author just promises not to do it again and nothing else happens.

Don't do it.  Just because unethical conduct has no direct consequences doesn't mean it's okay.  Don't even do it if your coauthors tell you to do it.  Note that the coauthor's permission is needed in addition to their knowledge.