Ivor, were you in Yorkshire by any chance? Only at my parkrun the other week, we had two people who don't know the course make a mistake and cut quite a chunk off, by running the second lap the same as the first.
I'm a member of a club. I run with them on Mondays, Wednesdays and Fridays but I still go to parkrun most weeks. I also do races regularly, with my club, with parkrun people and on my own. I don't think being a club runner precludes you from enjoying parkrun. Some of the fastest runners in my club (not me at all!) do parkrun regularly.
As for the 50/100 thing (there's nothing for 20, although juniors get a 10 t-shirt) is more of a personal thing for me. Although I'm wearing a red shirt next week as it's a 50th run for someone.