Griefing is the act of intentionally ruining other players' experience with a game. Seen in almost all co-operative multiplayer games, it entails a variety of acts aimed at causing one's own team to lose or making the game not fun for opponents.
Griefing in the Super Smash Bros. series generally consists of:
Griefing was especially common in Basic Brawl, as the combination of anonymity, complete lack of control over the match rules, free for alls being mandatory, and no restriction over stage and item selection, created conditions unfavorable for serious matches and ripe for griefers to abuse. This is one of the main criticisms players had of Basic Brawl, as many found it unenjoyable for those choosing not to resort to griefing themselves.
Unlike Brawl's wi-fi mode, which was entirely unmoderated, the one in SSB4 allows the reporting of griefers, which can result in them being banned from playing online.