There is a well known relationship between pupil dilation and multiple cognitive processes such as attention, surprise, task difficulty etc. It remains unclear which specific process modulates pupil dilation, but it is a fairly large (but slow) effect that can easily be picked up by consumer-grade eye-trackers. See below an example of a paper using pupil ...
Humans have loads of cognitive biases.
The one you have in mind could either be so called 'denomination effect', the tendency to care about large currency denominations and be less careful with smaller currency denominations. For example, not to care about the .99 part in a US 9.99 price tag.
It could also be the less-is-better effect. Small amounts of ...
The Stroop Effect is more for rapid-fire tasks and task switching.
Unconscious Thought Theory may be related.
Absent-mindedness can lead to automatic behaviors or automatisms.
Highway hypnosis is similar, where you don't remember the task you've done even though it's correct.
Otherwise, making a mistake due to habit/habituation?