Wallmaster, as several people before me have said. That thing scared the crap out of me when I originally played through OoT. After all, who expected that "monster on the ceiling" to turn out to be giant disembodied hand?
Then theres World of Warcraft gankers (players that kill you under really unfair circumstances, such as a level 70 killing a level 20 for those who don't play it).
When you know theres a ganker nearby but you want to give yourself the challenge of living, it suddenly feels like cat and mouse if your being chased but instead of outwitting some simple AI. Your having to out think another player hunting you down and while theres certainly no punishment for getting killed by another player. It can nerve racking at times waiting for him to give up or find you.