Stories crash because they can be overloaded with too many characters at times. This is especially true for a shorter than usual piece. For flash fiction it is generally recommended to have at most two characters (and often the flash pieces I read or write only have the one with other characters being referred to but never appearing). Because of the strict word count rules to flash, you can't go on at length and it limits how much you can say. It therefore limits how many characters you bring in.
Stories crash because the writer does it deliberately! (I'm sure that's the real story behind Pride and Prejudice and Zombies. I don't mind zombies and I love P&P but I do feel strongly those two worlds should never have combined!). There can be a lot of fun to be had here though. For example, what would happen if the big bad wolf from Little Red Riding Hood fame met Puss in Boots? Fur would fly - and not necessarily the cat's! (This is especially true if you have set in your head the idea of Puss in Boots as shown in the Shrek films!). The comic potential here is huge... have fun!