This is only an opinion, and no doubt others will disagree.
The
biggest cause of 'stuffing up a trick' in Public or in front of your friends is not putting enough practice into the effect BEFORE performing it to some friends.
So the ad said the trick was "Self Working"; it may well be that the mechanics of a trick are very simple but 'doing' the mechanics is only a small part of performing ANY EFFECT in front of friends or the Public.
Take the "21 Card Trick" (please - take it far away, and take 'Uncle Rufus' with it
little joke for 'magicians') It IS a self working trick, you can tell the spectator what 'their card' was: but baby, it ain't magic. The point is that any trick can be presented (performed - if you like) well or poorly. It can be made interesting or boring by only one person: YOU - the Performer.
IF you don't have absolute control of the mechanics, then some - if not most - of your attention will be on 'doing it right', not on your audience and not on your 'patter & presentation. Ask any of the 'long time' people interested in Magic and they will confirm this. YOUR Goal is to be able to do your mechanics without looking at your hands or the equipment (presumably a deck of cards) you are using.
Practice can be 'intelligent' or 'dumb'. Intelligent practice is done with specific goals in mind and ongoing steps and signposts for recognising improvement and attaining the required level of skill and competency for whatever you are aiming to master. 'Dumb' practice is simply the obtaining of a mechanical skill without any performance value. The "double lift" does lift two cards

but practiced unintelligently: there is nothing natural about it and the appearance of the mechanics screams out that you are "Doing Something",

even if they don't know what it is.
This leads to the second point I would like to raise. Some mechanical actions are just so 'fingery' that they are almost imposable to perform without some 'misdirection', to provide cover (to take the interest of your spectators away from what your hands are doing) for your execution of the 'Move'.
'Misdirection' is probably a slightly inappropriate word to use - but 'we' are a hide bound group. The Goal of 'Misdirection' is to direct the Audiences attention and interest to where we want it; so we can execute the 'move' out of the eye line and interest zone of the audience. Hopefully this will mean that 'they' will not see or register
our clumpy finger movements. I call it "The Dying Spider Finger Dance"! those of you who smiled know what I'm talking about.
"Misdirection" is an enormous Topic; both in the variety of methods we can choose to use and in the different ways we can use them. better people than I should be posting on it.
nuff said