Very light (<5%) radial engagement
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The use of high speed or trochoidal tool paths, that is, high axial depths of cut combined with low radial widths of cut or stepovers, typically 10% or so. When users experience chatter they keep reducing the stepovers from 10% down until they do not hear chatter.
With very small (<5%) stepovers, the cutting edges spend less and less time inside the cutting area. Hence, the cutting forces lose their sinusoidal shape and become similar to impacts (like grinding), which excite the modes of the system. This is chatter, but the cut is so light you cannot hear it. If you tap test an application like this, the Dashboard may show chatter at <5%. It is not incorrect. Again, the user can't hear it, but the surface finish will show chatter marks.
Long axial depths of cut, especially with smaller diameter endmills, cause a lot of deflection, are prone to chatter (heard or not heard), leave poor surface finishes and inaccurate walls.