How bad can an eyelash scratch your eye?

Minimal: Usually not bad. It can be very annoying and uncomfortable and should be removed. Usually symptoms resolve withing 24 hours.

Superficial: Ordinary eyelashes point outwards away from the eye. Sometimes an aberrant lash will grow towards the eye and this also is a common even from chemical or mechanical trauma causing backward growth. This creates a superficial, but uncomfortable partial abrasion of the cornea. It does not get deep enough to threaten the vision. Lid surgery can eliminate this.