I took a district-administered ACT today, and I erased a few answers and filled in a different bubble very darkly. The erased bubble was on the grayish side with the eraser I was using. In my experience with regular scantrons, they see both of them as marked and score it wrong. I assume the ACT grades with more sensitive equipment, but has anyone had problems with this? Is it known to be an issue?
The ACT advises students to "Erase any mark completely and cleanly without smudging" on multiple choice to avoid errors in automated scoring. Theoretically, this means there's a risk that the scantron will mark your answer as incorrect. It's hard for me to say exactly what will happen in your case because I don't know how light or dark the erased answer ended up looking. However, from what I've read in other forums where people have asked similar questions, it's very rare for scantrons to make these types of mistakes.
Provided the discrepancy between the darkness of the two bubbles is still relatively large, the machines used by the ACT shouldn't have a problem reading your answers correctly. If you get your scores back and are concerned that a mistake may have been made, you could consider ordering hand scoring for the test.