Time for a rambling old man story: One of my players is playing a champion, and at 3rd level he got the ability to deal a critical hit on a 19 as well as a 20. My brain reached back to 3E days, when criticals had to be confirmed, and so even though certain weapons had expanded "critical threat" ranges (19-20, for example), only a natural 20 was assumed to be an autohit, even though a natural 19 could still deal critical damage on confirmation. Butterfly Effect-ing my way back to 2014, I concluded that the 5E champion at my table still couldn't autohit on a 19, and that the feature only served to allow him to potentially deal critical damage on attack rolls of 19 as well as 20, but only if the total attack beat the target's AC. The champion's player protested this ruling, saying I was nerfing a cool ability that's important to his character. I decided to look into it. After some research and though, I concluded that 5E is meant to be quick, simple, and straightforward. Champions are supposed to be top-notch melee warriors, the kind who might score critical hits twice as often as other characters. I realized that this ability, while powerful, was hardly gamebreaking. Moreover, I realized the costs of my initial decision: I was spoiling my player's fun, and I was creating (marginally) more work for myself by complicating the hit-resolution mechanic. The cost-benefit of banning champions for autohitting on 19 just didn't work out in my favour, and so I reversed my decision and apologized for my foolishness, as well as explained why and how I came to that point. Three months later, the champion is still going strong, critting all over town, and my game is more fun than ever.
The point of my story is that sometimes, even we experienced DMs can make bad gut-decisions. Being awesome with mage hand is the whole point of being an arcane trickster.