Can a university revoke one's degree for his minor misconduct?
This depends entirely on what the misconduct was and the university in question. One extreme case is Jan Hendrik Schon where he published many articles using fabricated data. The University of Konstanz revoked his degree after the subsequent investigation of academic dishonorable misconduct. For something like this, the institution that had initially conferred the degree may revoke the degree despite the individual having already graduated.

If the individual has graduated, but has not yet been issued the degree, and depending on the severity of the misconduct, the university may also withhold the degree until further notice.

Overall, it really depends on the university's judgment on how severe the misconduct was. There are no universal rules regarding how the university ought to punish a student based on the misconduct (e.g., some universities are much more strict about plagiarism or cheating than others).