A druid suffering the Death glitch.
The Death glitch was a commonly observed glitch prior to the RuneTek 5 game engine upgrade. It involves the animation on a player or non-player character in the death animation position when it is actually alive. The bug was a client-side error, and as such, was not fixed until the game engine was upgraded on 2 September 2009. Players were only occasionally subject to this glitch, often in high-mortality locations such as Castle Wars, TzHaar Fight Pit, Duel Arena, and Pest Control. Often, the affected player would not know that they are affected and only a select few people would be able to see them on the ground.
NPC's that were frequently killed (such as guards, trolls, dagannoths, and Pest Control monsters) commonly suffered from this glitch. Attacking the affected monster would engage it in normal combat. However, the player could walk away from where the affected monster appeared to be during the fight. This was because the monster was actually walking around, but the glitch prevented the player from seeing where it really was. As a result, the player would approach and fight the monster on its actual position within the RuneScape server, even if it did not appear at that position for the player.