From Wikipedia:

At the Battle of Kalka River in 1223, Subutai's forces defeated the larger Kievan force, while losing the battle of Samara Bend against the neighboring Volga Bulgars – one of the Mongol's few, if not only, utter defeat; the Khwarizmi historian al-Nasawi says only 4,000 survived.[30] The Russian princes then sued for peace. Subutai agreed but was in no mood to pardon the princes. As was customary in Mongol society for nobility, the Russian princes were given a bloodless death. Subutai had a large wooden platform constructed on which he ate his meals along with his other generals. Six Russian princes, including Mstislav III of Kiev, were put under this platform and crushed to death.

"Bad ass of the week" is another site that mentions this along with another imaginative instance involving the Persians:

After defeating his ancient enemies in China, the Mongols turned West. The Russian and Persian Empires fell within a couple of years. When Baghdad was captured, the Mongols wrapped the Sultan of Persia and his family up in a giant rug and then trampled him with their horses. When the Mongols conquered the Russian steppe and stormed Kiev, they put wooden planks over the bodies of captured Russian generals and princes and slowly crushed them to death by eating dinner on top of them.

I have also seen this mentioned in a novel that I am reading. It sounds like a really bizarre form of torture. Did this really happen?