I Hate Snatch Steal
Attempting to summon number 99 with its 'discard 1 rank-up-magic spell..." effect while you control xyz override causes number 99 to be summoned in another monster zone instead of by using number 39 as a material. I also got an error message something like the following: "utility.lua 240 attempted to index a nil value

edit: I have had trouble recreating this glitch but it did happen at least once when I was using a deck and settings made to recreate it.

edit2: So far I have had this glitch happen once in LAN mode, but I was using a fan made card with the same kind of effect in place of xyz override so I'm guessing that the replay of that duel doesn't count (though I do have it saved), and once against the AI with 2 decks made to trip this glitch both made of official cards only. Sadly I don't have a replay of the 2nd instance but it DID happen and it had the same message.
This is a known issue and is being looked into.

Please use the search next time.