Isolate the input first
Unexpected movement can come from a held key, controller stick drift, Steam Input, an accessibility binding, or a current-game issue. Reproduce it in a safe area, then test one input path at a time.
1. Disconnect or disable controllers and test keyboard and mouse only. 2. Review in-game movement and accessibility bindings. 3. If using Steam Input, test the default layout and a clean restart. 4. Record the game version, device, layout, and reproducible steps before reporting it.
Do not delete saves or configuration files as a first response: those actions do not establish the cause and may discard useful evidence.
Source and version note
This page fills a dedicated technical topic indexed by shroomgloom.org. Its diagnostic order is deliberately version-safe and does not claim a single confirmed cause.