I have seen a few over the past 55 years - I still clearly remember one incident at the famous `Hut Dojo` around 1959 - a young guy did a forward breakfall, with his forward arm, he placed the palm of his hand first on the mat - taking his weight on his hand - suddenly there was blood everywhere - we were amazed to see that the centre of his palm had split wide open, he was taken to hospital for many stiches. I have never heard of that kind of injury before or since.
Sensei, quite an incident. But I'm not clear what happened.
When I do forward ukemi--especially a sort of long, diving ukemi, I put the "other" hand on the mat but I don't put any weight on it at all. I use it as a sensor for the distance and speed, so that the rest of my body can adjust appropriately at the proper instant.
I had a guy once--a really terrible teacher of ukemi--tell me how DANGEROUS it was to touch the mat with my other hand. He said, "Your arm will be driven out the back of your shoulder!" Of course, in my decades, I never experienced that. He was looking at my technique but not seeing. And soon after he said that, a young woman following his ukemi advice injured her shoulder.
One of the worst injuries I ever had was a punch to the mouth that actually drove my tooth through my lip. Had to have some surgery for that one.
Another time, a teacher "ground" his forearm across my lower face and crushed some saliva ducts in my lip, causing a large cyst that also required oral surgery.
I've had lots of arm and wrist strains and my elbows don't like to extend too far now, lest someone put another ikkyo on me...
Otherwise, thank God, very few problems.
Best to you.