[Guards]

I tend to believe that the people who use 'dirty' tactics while training are the people you should avoid training with. Their mentality even in training is not to improve their technique, even through defeat. Their mentality tends to be 'Victory at all costs', and the dirty fighters are the ones you can almost guarantee become the most offended or disturbed by a loss.

There are some legitimate tactics that 'feel' dirty sometimes though. These should be brought to the attention of your opponent if you are uncomfortable with accepting their application. In wrestling, Judo, Jiu-jitsu, etc. one of the common techniques is to apply heavy leverage and weight from a top-position to gain or maintain dominance. In some cases, this includes using your forehead/cheekbone/shoulder as weight, and can be quite uncomfortable to the receiver, especially if they have other minor injuries from training(a bruise on the cheek can be aggravated quite a bit by someone levering their forehead on your cheekbone with most of their bodyweight)