They say that love is blind, and it turns out that may be more true than ever. A 29-year-old guy in England lost part of his vision after having too strong of an orgasm. While cumming so hard you go blind may seem hard to believe, there's a legitimate medical reason for it.
First, the patient didn't exactly go blind, but his vision was legitimately impaired. The man went to the hospital after waking up with vision problems, and admitted to doctors that he'd had "vigorous sexual intercourse" the night before. Upon examination, the doctors found that he had a tiny hemorrhage in his left eye that was blocking part of his vision.
The problem, doctors assumed, was that while having an orgasm, the man had performed the valsalva maneuver—basically, holding your breath and straining your abdominal muscles, which sometimes can prevent you from climaxing—and the resultant pressure buildup popped a blood vessel. The case study published in the BMJ notes that his condition resolved spontaneously (i.e., just wait and it'll go away), but it's still a pretty freaky thing to wake up with after a long night of sex.
Watching your blood pressure probably isn't a bad thing during sex. Orgasms rarely cause permanent damage, so you can't really have too many, but straining too hard can, as we've just seen, sometimes overdo it.
There are plenty of ways to make sex even more exciting without holding your breath and clenching up. It's possible, for instance, for men to have multiple orgasms, but be forewarned: some pretty strange stuff can still happen; crying, sneezing, spontaneous weakness, and even headaches are all common side-effects of getting off. Still, despite the age-old tales, you won't go blind from masturbation itself—just be careful of those blood vessels.