I fell off the other day trotting and I remember banging my head, but I felt on my hip/side. Once I got moving again, I asked my instructor if my helmet needed replacing and she said no it was fine, I didn't land on my head. I did find out that I cracked a couple ribs, so I hit the ground harder than I had thought.
So I thought it wouldn't need to be replaced, but now that I know I've cracked bone it makes me worried maybe my helmet does need replacing.
So I just thought I'd see what the opinion was on a 'bad fall'.