I've experimented a lot with Missingo (Caught tons, and released a lot.) One weird thing I've noticed is that if you leave it in the daycare, something weird might happen. This doesn't happen all the time, but this is what you do:
1. Leave Missingno at the day care with the day care man.
2. Right after that, talk to the man again. Notice that now he pauses for a little while before answering.
3.Sometimes he says the usual, "Back so soon, your pokemon needs to stay longer." But other times he will say something like, "Your Missingno has grown a lot! It has grown 245 lvls (or something like that). You owe me $9999 (or something like that) to get it back."
4. If you take Missingno back, it will be lvl 100. I don't think anything really happens except the daycare man pauses a little now.
Also, when I went to the Elite Four and beat it with Missingno in my party, it made my Missingno look like Eve, my flareon in the Hall of Fame.