General What’s the most "illegal" feeling move that is actually allowed?

[quietoffensive]

The most illegal-feeling move has to be hiding my artillery behind trees while baiting the enemy units. A friend did it to me in Company of Heroes. Everything looked normal until, half of my army disappeared. It was so painful. The annoying part is that it was fully allowed they see as illegal. Some tactics just feel disrespectful even when they’re fair.