Which of the following is a common mistake in stage design?

Ignoring audience sight lines is indeed a common mistake in stage design. Effective stage design requires that all audience members have a clear view of the performance. If sight lines are not considered, certain areas of the stage may be obscured from view, which detracts from the overall experience and can lead to confusion about the action taking place. By failing to take into account how sight lines vary depending on the layout of the audience seating, designers run the risk of isolating parts of the production from viewers, thus diminishing engagement and impact.

The other options pose challenges but are not as fundamental as sight lines to the overall effectiveness of stage design. While using too many props can create visual clutter, it doesn't inherently interfere with the audience's ability to see the performance. Similarly, focusing on a single color can be a deliberate design choice that emphasizes mood or theme, and overcomplicating the set, while potentially distracting, does not necessarily negate the necessity of considering how every viewer can engage with the performance. Overall, prioritizing audience sight lines is crucial in ensuring that the design fulfills its primary purpose—enhancing the audience's understanding and enjoyment of the performance.