this is the usage I saw: 
http://www.news30over.com/archives/6613298.html
"中国の反日デモが韓国サムスンを襲撃！！サムスン大炎上！"
"405: わかってやっていたとしたら、こわい"
"418: >405 ボーナスステージだからな、中国１００都市に伝播し公安も対応できな
い 好きなだけ暴れられて、好きなだけ盗めて、絶対捕まらないなら 誰でも何でも襲う
に決まっている　ひろゆきもそー言ってる"

"A bonus stage (also known as a bonus level or bonus round) 
is a special level within a video game designed to reward 
the player or players, and typically allows the player to 
collect extra points or power-ups. Often a bonus stage will 
have no enemies or hazards, or may contain them but the 
player character is invulnerable to attack from them and 
cannot be harmed."
http://en.wikipedia.org/wiki/Bonus_stage

What does it actually mean? Is it something in a game?

870k