I caught one, along with a Snorlax over level 100. I leveled up Snorlax with Rare Candies till it's HP was beyond 1000. Then I defeated a Level 0 Missingno (yeah, they might appears in Level 0), got 0 Exp.Points and my Snorlax leveled "up" to Level 97 instead of 100.
I never got problems with my save file while catching Missingno, but it crashed Pokemon Stadium 1 and 2 when I tried to select it
Also, when you catch Missingno, your Hall of Fame screws up, and shows random Pokemon instead of the ones you used to beat the Elite Four. I remember doing this until Mew appeared, which was the only Pokemon I didn't have at that time

