What is edge case?
An unusual input that the guidelines do not clearly cover. Edge cases are where most labeling mistakes, and most quality checks, happen.
An edge case is an item that sits at the boundary of a project's rules: an image where the object is half hidden, a prompt that is partly harmful and partly fine, a review that is sarcastic. The guidelines usually cover the common cases well and the edge cases poorly.
Why they matter for your score
Platforms know that easy items tell them little about a worker. Gold tasks and reviewer spot checks tend to focus on the hard items, so how you handle edge cases has a large effect on your quality score.
How to handle one
First look for the closest rule in the guidelines and apply it consistently. If the project has a list of examples or a FAQ, search it; edge cases are often answered there. If nothing covers it, follow the instruction for unclear items (many projects have a flag or "cannot judge" option) and say in your comment what made it unclear. Guessing silently is the worst choice, because it cannot be corrected.
Example
A bounding box project says to label every car. A photo shows a car reflected in a shop window. The project FAQ says reflections are not labeled, so the labeler leaves it out.