ToME: the Tales of Maj'Eyal

Tempest peak, though it has happened in other dungeons I've tested it in, such as the maze.

My archmage has a level 4 (effective 5.2) teleport spell. While using it in a dungeon to escape a mob, it fizzled and teleported me straight into the boss room. It was close, but I managed to escape. Now I'm paranoid about using teleport if it's unreliable. What could cause it to fail? I've checked ...