We encounter cannibalism in the western middle ages infrequently, and when we do it is obliquely, as though from the corner of our eye, just beyond vision.

A capitulary of Charlemagne at the end of the 8th century makes cannibalism a crime punishable by death and equivalent to sorcery. The blood libel attached to Jews in English 11th century has cannibalism at its core: a Christian boy supposedly kidnapped, his blood consumed. Between these events we have inklings of cannibalism in several times of famine.

But then there is Ma'arra an-Numan 1098. After the long yet successful siege of Antioch the crusader army provisions were low and raids to the surrounding areas led to the siege of the city of Ma'arra.

Fulcher of Chartres travelled with crusaders and wrote about Ma'arra in 1100 :

When the siege had lasted twenty days, our people suffered excessive hunger. I shudder to speak of it, because very many of our people, harassed by the madness of excessive hunger, cut off pieces from the buttocks of the Saracens already dead there, which they cooked and chewed, and devoured with savage mouth, when sufficiently roasted at the fire. And thus the besiegers more than the besieged were tormented. (Historia Hierosolymitana)

Raymond of Anguilers, chaplain to lead crusader Raymond of Saint Gilles, wrote of the same event within a few years:

Meanwhile, there was so great a famine in the army that the people ate most greedily the many already fetid bodies of the Saracens which they has cast into the swamps of the city two weeks and more previous. These events frightened very many people of our race, as well as strangers. On this account very many of us turned back [...] But the Saracens and the Turks said on the contrary, "And who can resist this people who are so obstinate and inhuman, that for a year they could not be turned from the sieg of Antioch by famine, or sword, or any other dangers, and who now feed on human flesh?" (Historia Francorum qui ceperunt Ihersalem)

The chronicles of the Christians on spiritual mission to the holy land, the crusades, thereafter is dotted with other accounts of cannibalism, often explained as due to hunger. Or recognizing the depravity, a chronicler will refer to 'unmentionable' acts.

Subsequent writers, such as Guibert of Nogent, begin to transform these Christian acts of cannibalism into the work, the fault, of a subset of crusaders: the poor.

Was cannibalism common? No. But it happened in the least expected places.

For the connection of the crusaders events to the appearance of cannibals in King Arthur legends, see the fascinating (if arch) book by Geraldine Heng, Empire of Magic: Medieval Romance and the Politics of Cultural Fantasy (Columbia University Press, 2003)