Many years ago, my then GF was sucking my cock, and I was getting close. She had previously told me not to cum in her mouth. She could tell that I was close and pulled off and told me that if I did cum in her mouth, she'd bite me. Well she was giving me such a good blowjob that I just couldn't pull out. I then did not pull out and did cum in her mouth, she was true to her word. As I was still in my orgasm she bit my cock and damn did that hurt!
Ever since then I've had a problem cumming from being sucked.
Thinking about it now, I suspect that she may have actually wanted to hurt me and set it up that way.
On a happier note a different partner sucked me to two orgasms without stopping and did not bite me... that was very nice.