I was recently on a UA flight from EWR to LAS. The following flight from the gate we boarded at was a UA flight to BOS. To make a long story short, passengers for the Boston flight boarded our plane, which caused confusion when two people had the same seating assignment.

With scannable boarding passes, how can it be that both people for the LAS and BOS flight successfully boarded the plane? Does this happen due to human-error, e.g. ignoring computer warnings when a wrong boarding pass is scanned?

Whenever I get a last-minute upgrade, there is an an audible sound and red-blinking light which cannot be overheard/overseen, to indicate that the agent needs to give me a new boarding pass.