It happened to Anderson Silva and we just saw it happen to Tyrone Spong in Glory aganist Gohkan Saki. Both of these champions suffered from shin breaks throwing hard low kicks into their opponents block. Throwing a powerful kick at the right angle into a knee can cause this. So, when throwing low kicks set them up first. Let them come behind a hand technqiue or catch them while moving. Here's a clip of Spong's recent injury.