Started in North London at crazy house parties. It is when stuff is broken or moved around or just generally mucked up in people's houses. It is not necessarily vandalism, just annoyances e.g. throwing an ironing board over a fence, or rearranging all the cupboards in the kitchen.
Also there is a varient called emotional mischief, whereby harsh rumours are spread about a particular person.
When mischief starts, the word "mischief" is chanted and the group partaking in the mischief charge around the house. It can also be done subtly.
Putting the contents of the larder in the oven
Spreading rumours about how x described y as a Nazi C***.