Never slap Nigerian striker Frank Temile during training.
Dynamo Kiev's Dmitry Korkishko learned this lesson the hard way when he landed an open palm on his teammate's face during a practice session in the Ukrainian capital this week.
Temile swiftly responded with a brutal left hook, which quickly escalated into a full-scale fisticuffs fracas. The Nigerian was so enraged that he has to be pulled away by his ankles, which is always a good sign of a tenacious beatdown.
It's the kind of intense training ground violence that John Hartson would be proud of.