The image on the left captures what would be seen by those with astigmatism, which, as Unusual Facts suggest, relates to having a slightly curved cornea.
For those unfamiliar with the condition, the NHS defines it accordingly: "Astigmatism means your eye is shaped more like a rugby ball than a football, so light is focused at more than one place in the eye. This can cause: blurred vision, headaches and eye strain."
Essentially, the curves of the cornea on the front surface of the eye and the lens inside it are mismatched, meaning incoming light rays do not refract as they would in the case of a perfect eye. Instead, light strikes the retina at multiple points, creating a blurring effect.
The condition is most commonly present combined with another refractive error like short-sightedness or long-sightedness.
The reaction to the lens flare tweet was largely one of surprise:
@UnusualFacts6 Wait people can see lights normally? I thought everyone saw those lines-