A ledger line drawn for one voice must not collide with a notehead or stem belonging to another voice sharing the stave

In a stacked or SATB texture where two voices share one stave, a ledger line drawn to reach one voice's extreme note extends across the FULL horizontal width the ledger convention specifies (see ledger-line-geometry-length-thickness-spacing) — which can pass directly behind or through a notehead belonging to the OTHER voice if that voice's note falls at a similar x-position and pitch level. Engraving practice requires the ledger line to visually read as belonging to its own voice's note without appearing to strike through the other voice's notehead.

An SATB alto note requiring a ledger line below the treble stave (e.g. an unusually low alto G3, at the boundary needing 0-1 ledgers) where the soprano voice's rest or notehead at the same beat sits directly behind the ledger stroke.

Ross, The Art of Music Engraving, and Gould, Behind Bars, both discuss multi-voice ledger-line collisions under their broader multi-voice notehead-collision guidance (Gould's 'Voices on One Staff' chapter cross-references leger lines as one more shared-stave collision surface, not a separate rule set).

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.