I remember the end of the war in my last career. I had essentially lost track of the date and ignored a radio message since I figured it would be the usual "Keep up the good work" pep talk messages from good 'ole Karl.
So while patrolling, I encountered a C-Class Destroyer off the coast of France. I dove immediately to periscope depth and sped to his position. I got really close but he had not detected me yet despite me being at full power. I figured "What the hell", ordered a torpedo attack and sunk him.
Then I surfaced and decided it was a good time to check that radio message. Apparently the war had been over for two days now. Whoops.