February 19, 1859
Congressman Daniel E. Sickles is acquitted of murder due to a temporary insanity defense, the first time in US history this defense was used successfully
Sickles — who had been having a very public affair with a known prostitute, including taking her on vacation to England while his pregnant wife was left at home — shot and killed district attorney Phillip Barton Key II for having an affair with Sickles’ wife.
Sickles publicly forgave his wife for her dalliance after his trial. The general public was seemingly more outraged at Sickles’ forgiveness than the murder of Key or the historic defense and acquittal.
On a side note, Key was the son of Francis Scott Key, the author of the lyrics to The Star Spangled Banner.