You could certainly test this by making completely random moves and seeing whether it's more likely to make illegal moves in those positions. Though I think yo

You could certainly test this by making completely random moves and seeing whether it's more likely to make illegal moves in those positions. Though I think you're overestimating how many positions have occured. Frequently, by move 20-25 you have a unique position that's never been played before (unless you're playing a well known main line or something)