Spiral of death
The failure mode where a fixed-timestep loop that cannot simulate a frame in time keeps demanding more catch-up steps each frame, falling ever further behind until it freezes.
A fixed-timestep loop banks elapsed time in an accumulator and runs one simulation step per banked timestep. If the step itself takes longer than the time it represents — a heavy frame, a stuttering machine — the accumulator banks even more time, which demands even more steps next iteration, which take even longer. The spiral winds downward until the game stands still.
The cure is to clamp the maximum steps per frame: better to slide into slow motion than to freeze outright. The failure and its fix were named in Glenn Fiedler's article "Fix Your Timestep!" (2004).