A little fanfare?
Ok, got a contributor page. Let's try this again.
The trope of the day is...
Okay, that's something I can work with. To sum up this trope, whenever somebody says this phrase, they will indeed be stopped within the next five minutes of screentime. But the vast majority of people already know this. I mean, the minute that you hear a villain say this, you know that failure is a forgone conclusion. It's practically a message from the writer's straight to the viewers saying "Yeah, he's going to lose".
Yet it keeps popping up. But why?
Well, we know that the vast majority of plans fail horribly. Robert Burns pretty much hit the nail on the head when he wrote "To a Mouse". I have no idea what this poem means, but that's okay because the only part that actually matter is the lines "The best laid schemes o' mice an' men / Gang aft agley". You might recognize it as the inspiration for the title of "Of Mice and Men".
But anyway, to those of you who are like myself and don't have any idea whatsoever what "Gang aft agley" is supposed to mean, there a much more pithy and not opaque summation of the same basic idea that you will probably recognize: Murphy's Law. You know how it goes. "Anything that can go wrong, will go wrong." And the audience expects this to carry over into media. This makes sense; if everything always went perfectly according to plan, every movie ever would be really boring.
Five minutes later, Bond was cut in half, and Goldfinger was well on his way to becoming even richer.
That might work as brief parody, but there's no way you could make a good movie out of it without adding something. Of course, this only explains why the plan fails. It doesn't explain why a character decides to go and declare how perfect their plan is before it falls apart.
The only reason that I can think of is that we like to watch prideful people get taken down a notch. When somebody displays enormous hubris, we like to see them suffer for it. We really like it. There's even a word for it. And let's face it, seeing somebody gloating is one of the most annoying things ever. Even C.S. Lewis pointed out that "every one in the world loathes when he sees it in someone else." Heck, in ancient Greece, hubris was a crime! Nobody likes pride. There's a reason that it's one of the Seven Deadly Sins.
But that's really the core of this trope; a combination of the fact if the plan goes perfectly, nobody wants to watch it (In general, at least. I'm sure there are exceptions) and that we enjoy seeing somebody's failure more if they've been gloating about it. I know that I feel much less guilty about laughing at somebody's failure if they've been high and mighty about how perfect they are.
Of course, this sort of thing always happens in real life, too. The moral of the story? Plan for failure and then when nothing goes according to plan you'll actually make progress. Unless planning to plan for failure counts as a plan in itself, at which point you're screwed either way. Oh well, just don't brag about your plan and you've got a decent chance of success.