Carpet Mod seems to mixin to the same place to make BEs movable, so it's incompatible with PistonBlockMixin from this mod. See the following crash log: crash-1.log

This occurred with Piston Control 1.2.2 and Carpet Mod 1.4.0 on Fabric Loader 0.9.1, Minecraft 1.16.1

I'll look into conditionally applying mixins - though, does carpet still apply its mixin if its movable BE module is disabled?

I think it does, because the module can be enabled and disabled at runtime (which can't be done with mixins)