As the readers age, they become less reliable and more prone to error and breakdown. To rebuild them, elections companies use a dwindling supply of spares, hangar queens and whatever rebuild protocols they devise. However, sometimes two cards are sucked through. This is probably because pre-punching makes small ridges on the bottom of the card, and an identical pattern of small troughs on the top. The ridges tend to get caught in the troughs.
Detached chad can become stuck in the feed path, increasing double feeds and misfeeds.
Detached chad can jam two cards together, increasing misfeeds. In some machines, detached chad can jam over the light or sensor, causing holes to not be read until the chad blows out of the way. Detached chad can migrate from one card to the next. Any damage a card has sustained can confuse a card reader or cause it to malfunction. People are better able to deal with such problems.