Wild Magic sorcerers (PHB, p. 103-104) have the Wild Magic Surge feature at first level:
Starting when you choose this origin at 1st level, your spellcasting can unleash surges of untamed magic. Once per turn, the DM can have you roll a d20 immediately after you cast a sorcerer spell of 1st level or higher. If you roll a 1, roll on the Wild Magic Surge table to create a magical effect. If that effect is a spell, it is too wild to be affected by your Metamagic, and if it normally requires concentration, it doesn’t require concentration in this case; the spell lasts for its full duration.
One of the 50 effects of the Wild Magic Surge table (on a roll of 11–12) is the following:
Roll a d10. Your height changes by a number of inches equal to the roll. If the roll is odd, you shrink. If the roll is even, you grow.
Let's say that a Wild mage "spams" Wild Magic Surges (with a very Surge-friendly DM who always says "Yes" when there's a possibility for a Surge), and rolls the above effect many, many times, and then always rolls an odd number (height decrease). This can technically cause him to be… 0 feet tall, as there's no written minimum for that Surge.
Compare this to the wording of a similar Surge (on a roll of 35–36):
Roll a d10. Your age changes by a number of years equal to the roll. If the roll is odd, you get younger (minimum 1 year old). If the roll is even, you get older.
But on the height change Surge, there's no height minimum written. So… what would happen there? Would the Wild Magic sorcerer vanish from existence?
Of course, this situation has a low chance of happening, considering that if you roughly get every number of a d10 an equal amount of times, you'd have an average decrease of 25 but an average increase of 30 (so a net increase of 5), but it's still possible to be very, very unlucky. For example, the smallest default height a player character can have is 27 inches (by being a kobold and rolling/taking the lowest height modifier), so a few consecutive odd rolls and poof, your height is 0. What now?
Best Answer
For the sake of sanity, add a minimum threshold
Things get way too awkward even before zero - at 1 inch (~2.5 cm) and 150 pounds (~68 Kg), the character is now made of something with a stupidly high density. Note that the mass doesn't change with wild magic, so it doesn't vanish - it still has mass. As something can't occupy a zero volume and still have mass, either we completely kill physics to its core (okay we've been doing it already, but this is where I draw the line) or we don't allow it.
If someday this actually happens, I would simply say that, under X inches (possibly around ~15), the wild surge simply doesn't affect the creature any more.
This is not RAW, and it does not need to be. This is a situation that is probably never going to actually happen in a table, so it was probably not thought in advance when making the rules.