the REAL worst injuries I ever saw was when going with a rescue team to the area devastated by the Gölcük earthquake in 1999.
Apart from that, the worst injury I ever saw was a breakfall from a bike. I was coming by bike from shopping (we were making vacation in a small village on the Black Sea coast), and out of the blue came a young guy on his bike, it was a very small and not steep hill, and he just crashed into NOTHING two meters in front of me, flipped over his steering wheel (you say so for bikes?) and crashed frontally on the right side of his face. He gave one horrified look and passed out. There must have been a very small hole in the street or any other unevenness he hit with the wrong angle; he completely lost control without any visible reason.
I just managed to stop my bike before crashing on him, and when I bend down to him, he was unconscious, bleeding everywhere, a tear trickled from one eye, and he lost a lot of teeth on his right side. Another passer-by called an ambulance, but since the village was so small it would take some time until the ambulance came. I sincerely thought the guy was dead, but he came back to consciousness. Then we stopped a car and asked the people if they could bring him to the hospital - and they didn't want to because he would bleed on the seats. One of the bystanders started to curse their heartlessness, and then they took him grudgingly to the hospital. Until then he could stand again but was completely numb and couldn't speak.
That was the worst I saw on the street. In the dojo I had once an accident with ACL rupture, but that was well repaired and doesn't make any problems anymore. I once saw a guy wanting to show off making a breakfall over a large gymnastic ball, crashing with the shoulder into the tatami and dislocating his shoulder. He had to go to hospital immediately and couldn't train for some weeks.
And we had a newbie breaking a rib on one of her first lessons. she had paid for a year but was scared off. All in all, I think that aikido is pretty safe compared with other sports or with daily life
All the best,