You can alaways do a mistake, even at things you are excellent at. Just compare professional basketball players doing free throws at practice (were they are hitting dozens in a row) and during a match (where hitting above 80% is considered good).
If people claim that they never fail things that they think they are good at they don't live in reality or are not honest to themselves. Besides making a mistake sometimes random things happen that distract you or interfere. Is it 1 in 20 times you do something? Maybe not. But none of the D&D rules are realistical to that level of detail. There is a reason most systems have this rule and actually One D&D seems to be adding it to the next iteration of the ruleset.
