Imagine the situation. You're playing happily, completing quests or dungeons. Suddenly, you decide to change your class skills. You approach the NPC, make the changes, and realize you've made a mistake. You pay the NPC 1500 gold, but nothing changes. You're perplexed. Then, you notice a message on the top right corner of your screen that reads "3/3 skills selected." This means you'll need to level up your current class ability. It's a shame, and you're upset.
And now I realized that patch No. 46 brought a mechanic where one mistake is worth a lot of time.
You're wondering, "Didn't the developers think about the errors?". The banal human factor