Oh, hey, I've programmed gameboy before. I can field this one. Although it's actually a general programming thing, really.
There's a defined start of the 'pokemon' data, and each pokemon's data is in there, in a very organized fashion, one after another. As it needs to be accessed, the game looks (pokemon number) * (size of one pokemon's data) past the start of the pokemon data, and it finds the information it needs.
Trouble is, the game is on an 8-bit, so the highest number is 255, not 151. And when you're loading from something that wasn't supposed to be used to load this data, there's no guarantee it'll be in the range 1 to 151. So often, it's just.. Not. As is the case here. But data for things that aren't pokemon is what's after the pokemon data. Not looking at the code, I couldn't tell you what that data is, but it's not pokemon data. So the game looks for pokemon data well past where it actually exists, and as a result gets data that is for all intents and purposes completely random. Predictable, but useless data.
So they're not 'placeholders,' exactly, they're just.. There. It's like if you had a book that was so many pages long, and you tried to see what was in the book a few pages more than that in. That's part of the hardcover, which isn't paper and doesn't have text. But if you could somehow look at that part.. Say, you cut open the cover.. You'd see all the not-text that was there. And then Pokemon Blue takes a look at it, says 'that looks about right,' and declares that not-text to in fact be text and reads it, declares it a pokemon, and shoves it on to your screen. And then your game boy (/ emulator) responds with a 'What the hell did you just put in me' and spazzes out.
Because it's not supposed to be move data. Why would it have 100% accuracy? That would be very unlikely, since it's effectively random. In this case, while HM02 does have 177 power, its accuracy is a mere 6%. Actually, every glitch move is terrible with the exception of TM05, which in Red/Blue increases the user's evasion three stages (use it once and everything's chance of hitting you is halved, twice and it's reduced to 1/3 its normal value), but is only learned in Yellow (where it has the different, less beneficial effect of freezing the game.) If you want to hack it on to one of your pokemon, its move ID is CD, or 205.