Unstable replays

An unstable replay is a replay which can produce different outcomes depending on how it is played back, as if the replay tape could split into diverging timelines at some specific frame. Unstable replays are a rare occurrence, but not vanishingly so, and typically are RH racing laps with aggressive powergear slides.

Whether a divergence will appear in a replay, and what it will look like if it does, depends on a large number of factors, including but not limited to: the chosen camera, in-game graphics settings, the number of cycles in DOSBox and the system (be it an emulator or real hardware, as the effect has also been observed in native DOS systems) used to run the game. Since, in contrast, reproducing a replay with only the bare game state loop (be it by fast-forwarding the tape or by using the repldump tool) gives consistent outcomes across systems, instability is believed to somehow arise from the graphics layer. Since live gameplay can't help but involve graphics, the exposure of drivers to instability can be limited only so far.

While unstable replays are by and large just one more amusing curio of Stunts, the conflicting outcomes can lead to confusion when validating laps in a race, specially given how the system dependence of the effect can make it hard to reproduce. That being so, this article aims at clarifying what to expect upon facing an unstable replay and how to effectively navigate in it, which should help avoiding, or getting out of, messy situations in competitions.