Any bar shorter than a full bar, other than the first (anacrusis) or last, must coincide with a repeat sign or a meter change

Outside the opening anacrusis and closing compensating bar, an incomplete bar is only legitimate immediately before/after a repeat barline, or exactly at a meter change. Anywhere else it signals a rhythm-arithmetic error.

A derived bar mid-chorale sums to 3 beats instead of the declared 4/4's 4 beats, with no repeat barline and no meter change at that point.

Gould, Behind Bars, ch. 'Bar lines and repeat marks'; Read, Music Notation, incomplete-measure conventions.

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