The most recent version of Mixin fixes an issue which prevents ReplayMod and SpongeForge from loading together. While Sponge shades the latest version of Mixin, ReplayMod's version ends up getting used since the ReplayMod coremod gets loaded first by FML.

If you could update ReplayMod's Mixin dependency to 0.6.8, it would be a great help to Sponge. Thanks!