The Beer Hall Putsch in November 1923 seems pretty serious. It failed and Hitler was eventually arrested and charged with high treason, but only given a 5 year sentence. While in prison he authored the first part of Mein Kampf and grew in popularity. Hitler ultimately only served little more than 8 months and was pardoned by the supreme court. Why was he pardoned?
More can always be said, but Hitler's brief time in prison and parole was covered in this older answer by /u/kieslowskifan which should be of interest to you.