Some of these were completely unforeseen, others in retrospect were pretty obvious, but at the time not so much, and others you have to wonder how they managed to make it live in the first place. We're going to talk about them now.
Vanilla WoW: The PvP ladder
Before the ladder, there was mainly world PvP. Spots like the Crossroads in the Barrens (close to a convenient neutral port so Alliance could get there easily) and Tarren Mill/Southshore were hotly contended for almost no good reason at all besides simple factional hatred and a desire from players to kill players. All of that changed with the introduction of battleground and honor rewards, the best of which required a player to achieve a certain rank to attain. What happened next was simple - some players hit upon a means to achieve that high PvP rank, namely, play in shifts.
The ladder was abused from the moment of its introduction. People formed groups who hit the BGs together, sure, but that wasn't the abuse part. The abuse came in the form of people sharing their account information and playing a specific character in shifts, literally keeping said character in the BGs for days at a time. If you were trying to play your character fairly, you simply couldn't compete with the five people who were playing that one warlock nonstop until it had all the high ranking PvP gear, and then shifting to the next player's warrior or paladin. I knew people who tried to stay awake for two solid days doing nothing but hitting up Alterac Valley and Warsong Gulch. It was painful to watch. The ladder ended up being removed before the end of vanilla, and it was the best change they could have made.