Stigmata seem weird to the modern person, even most devout Christians: marks that suggest the wounds of the Crucifixion appearing on a living person who isn't Jesus. And yet the stigmata were (relatively) common in the late Middle Ages.
The first person to display stigmata was Saint Francis of Assisi (d. 1226). The story was that Christ appeared to him on a mountain top and marked his hands and feet with wounds that looked like wounds from being nailed to the Cross. He was also supposed to have a wound in his side that would not heal. Francis, who was an extremely humble person (much more so than the Jesus of the Bible, who told people very firmly what-was-what), seems to have hidden these wounds, which kept on bleeding intermittently. His closest followers, however, found out about them, which is why we know about them now.
The above image is a picture of Francis painted about a century later by Cimabue. It shows stigmata.
Francis was considered a saint immediately upon his death, though it took another two years for him to be officially so declared (the current Pope Francis took his name from him). He was buried deep, because his followers were concerned that otherwise people might want to get a relic, a finger bone or toe bone or the like.
He was however exhumed in the nineteenth century during some renovations of the church at Assisi. The story is that he had bones looking like nails (though made of bone) protruding from the wrists of his skeletal hands.
Fact? Fiction? Hard to say about any of this, because we aren't there to see for ourselves. But in a way it hardly matters. What matters is if people believe and if they act on their beliefs.
In the centuries after Francis, a number of other highly religious people were marked with stigmata and frequently were declared saints. The vast majority of them were women. For example, Catherine of Siena (d. 1380), the other patron saint of Italy (besides Francis), was said to have received the stigmata. She also wrote chiding letters to the pope and brokered peace treaties between warring Italian city-states. Though stigmata have become far less common in recent centuries, there are still cases. Many are denounced as frauds, but at least one twentieth-century case was declared to have baffled doctors.
Declaring stigmata a result of hysteria or easy bruising sort of misses the point. Stigmata are supposed to show that the person is truly trying to imitate Christ, and that Christ recognizes that they are succeeding. Certainly some people have injured themselves deliberately in a pathetic effort to get attention, but if a religious person devoutly believes that her strenuous efforts to be holy have been recognized by God, it does neither her nor us any good to tell her that she brought it all on through hysteria.