A beam must never cross a barline, even when the underlying beat-group run is otherwise legitimate

Beat grouping (which notes get beamed together within a bar) follows the time signature; no beam may extend across a barline into the next measure, regardless of how the notes group rhythmically.

A run of eighth notes that starts in bar 2 and, due to a beat-arithmetic or barline-authoring mistake, continues to be grouped as one beam into bar 3.

Universal engraving convention (Gould, Read, Ross all state this without qualification); unconditionally enforced by Dorico, Sibelius, Finale, and LilyPond as a hard rule with no exceptions.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.