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Correctness with respect to the benchmark. A slow reference renderer could produce the target image, and renderers need to achieve either exact or close reproduction to the reference. Otherwise, you could just make substantial approximations and claim a performance victory.

Bezier curves can generate degenerate geometry when flattened and stroke geometry has to handle edge cases. See for instance the illustration on the last page of the Polar Stroking paper: https://arxiv.org/pdf/2007.00308

There are also things like interpretting (conflating) coverage as alpha for analytical antialiasing methods, which lead to visible hairline cracks.

I assume parent commenter means to avoid things like rendering the same pixel twice for adjacent paths, and avoiding gaps between identical paths. These are common problems for fast renderers that take liberties with accuracy over speed. (e.g. greater numerical errors caused by fixed point over floating point)