Be very careful
And some hurt way more than others. The act requires careful consideration for the organs involved and obviously any reasonable amount of caution.
1. Spicy tongue
This occurs when a partner performs cunnilingus or fellatio with a tongue laced in spices. They can have a very "hot" effect on the tender loins and one should therefore brush their teeth before engaging in oral sex after a spicy meal.
2. Stuck objects
Some couples decide to become adventurous and get- creative- with their sex toys. And some of them aren't "traditional" sex toys. So much so that Emergency Rooms in major hospitals have reported odd objects getting stuck in people's rectums. Such as potatoes. Ensure the correct sex toys are used, lube is sufficiently used and the anus is dilated enough.
This happens when excessive friction causes skin to break around the sex organs. When fluids run dry- lube is your friend.
4. Penile fracture
This happens when ambitious angles are attempted during sex as the boner can only break when it's at full mast.
5. Cutting off circulation
This happens when male partners use cock-rings. There should be enough room for blood flow when the ring goes around the erect penis.
6. Pressure problems
There have been reported cases of hickeys causing blood clots to form- so don't over do it.
There have also been instances where kissing caused air pressure to reduce in the mouth that resulted in deafness in one ear. Just take a break and breathe.