There were two instances in my campaign that I'm aware of.
One involved my teenage son, "creatively re-interpreting" die rolls, and just so happening to have the right arcane spell memorized for precisely the right occasion. A sit-down talk, a few threats, a witholding of allowance, cured that.
The other one was someone else in the campaign "misreading" the dice. I acted oblivious to it, and introduced the idea of witnessed die rolls (up til then we hadn't), cluelessly saying "It's just so much fun when we all see your triumph when you roll that nat 20!"
My die rolls stay secret because, as I've told the group, sometimes the results are modified by things they can't know about. Although, when I roll nat 20's against them, I show them those results with a quick screen-lift.