Edit Mode: Moving face/edge/vertex screws up mesh

I’ve no idea why this came up all of a sudden. I’m modifying an existing character into another one, and when I move certain parts in Edit Mode, an entire area of the mesh fucks up and moves around:

Any idea why? The object has no vertex groups or modifiers, or anything like that, and I experienced no problems at all while making the damn thing.

Ah, alright. Looks like X and Topology Mirror were active. Although why they were affecting a non-symmetric mesh like that is beyond me.