I always playtest my own games. Aside of ensuring stability, I need to test balance and pacing etc and of course to see if it is completable.
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Thanks for your reply DarkBloodbane. Of course I have also playtested my game so many hours, as you said for pace, balance, stability, etc, but beating the most difficult challenges in the game proved to be hard. And sometimes you set a trophy or a challenge, and you know it's possible, but very tricky to get. I'm thinking of people testing the platinum trophies in real conditions. They don't get it first go, especially on platformers and the likes. In my case, finishing individual levels in 1 life was easy, but achieving 40 of them in a row in 1 life is tough. But doable with enough practice.
You reminded me of challenge puzzle from my game Fire Leaf Water. There's a 10th level challenge in which player could complete normally or complete the star challenge. The latter requires scoring at least 400 or 500 depending on number on the star block. I could complete the other 10th level challenges but that one level really puzzled me.
I almost wanted to reduce the score requirement or whatever but I didn't give up and managed to beat the challenge. Therefore I am sure the challenge is doable.
That made proud of being able to create challenging puzzle which really challenged me ;D.