Well, you may have heard of this, it is more commonly known as "The Celebi Glitch", a certain "Pelliper(sp)" of SPPf alegedly solved it, but I have a few theories of my own, but first, here are some posts I've made about the issue;

Btw, when I post that TC cave thing on GameFAQs, some people said you were lying because you didn't properly define a ->[MOR]<- and I think they were right, SM, I could make one in paint as well, and lettme ask you one thing Pelliper; does your theory explain the Mildoc found on that route sometimes? I'm pretty sure that IF Serebii can be found there, it's not because of the reasons you stated, the most likely explaination is that it(if true) is caused by an in-game event altering a hex valuse to thaof Serebii's and so making it appear, MissingNo appears if a similar error or edit(like the Regi caves opening or the Eggs) causes a HEX value that is not of any pokemon in the game is created to prevent a fatal error that crashes the game, but if the HEX was edited or changed by an error to be that of Serebii, Serebii would appear, same with any other pokemon, and so toushe, I'll be suprised indeed if yoy can beat me now that I've gaied my current level of knowlage.
Well, not a bad reply, but the thing is, I acually have something thst could be condered proof of the HEX edits are the cause of MissingNo; All the routes MissingNo appears on accept the one in question have major coding edits in them, three of them have the Regi-caves and the other has the extra varriable that the eggs given at the daycare creates, but this route has no such thing that we know of accept the safari Zone and that really isn't a major extra varriable, but another thing is that many people found their Lati@s on the same route at least once, including me, it is possiable that the edit is that it is where the Lati@s are first coded into the game, ifo, that would explain both instances. PS: If it could cause Serebii to appear by inadvertently changing a HEX, it could make any pokemon appear, soes anyone here know if Mildoc and Serebii's HEX valuse is similar? if so, is there another similarly HEX pokemon on the route in question who's HEX could be the oneinadvrtantly altered?

Now to put my theory in plain words;
All the routes MissingNo appears on accept the one in question have major coding edits in them, three of them have the Regi-caves and the other has the extra varriable that the eggs given at the daycare creates, but this route has no such thing that we know of accept the safari Zone and that really isn't a major extra varriable, but another thing is that many people found their Lati@s on the same route at least once, including me, it is possiable that the edit is that it is where the Lati@s are first coded into the game, ifo, that would explain both instances. PS: If it could cause Serebii to appear by inadvertently changing a HEX, it could make any pokemon appear, soes anyone here know if Mildoc and Serebii's HEX valuse is similar? if so, is there another similarly HEX pokemon on the route in question who's HEX could be the oneinadvrtantly altered? I'm pretty sure that IF Serebii can be found there, it's not because of the reasons you stated, the most likely explaination is that it(if true) is caused by an in-game event altering a hex valuse to thaof Serebii's and so making it appear, MissingNo appears if a similar error or edit(like the Regi caves opening or the Eggs) causes a HEX value that is not of any pokemon in the game is created to prevent a fatal error that crashes the game, but if the HEX was edited or changed by an error to be that of Serebii, Serebii would appear, same with any other pokemon.

PS: Serebii of SPP has acted like a know-it-all about this many times, but he does not know all about the pokemon world even though he runs the world's top pokemon site, if he did, it would never be updated.
Well, think of me as you will, but if you do find Serebii/Celebi on route 123, remember to E-mail me saying it.