Lanefan
Victoria Rules
Not only have I seen it, it was my character that did it.So, dirty secret. In no edition of D&D, 1e AD&D through to 3e D&D, in 40 years of gaming have I ever seen a single classed M/U or Wizard survive past 6th level. I

In 3e, of all things, I ran a single-class Wizard (Illusionist) up to 8th level before she died. They brought her back, and she eventually got her 11th before her own party killed her.
What's incredible about this is that the character was of, well let's just say the unwise persuasion, and by rights probably should have died shortly out of the gate. And it's not like nobody else died; we had a fairly consistent turnover rate in that party to start with, then started reviving people once we could afford it.
I should also point out that in our games (1e or 3e) it's bloody rare for a character of ANY class to get to 6th without dying at least once. MUs don't have the monopoly on dying quickly.

I've seen numerous MUs go from raw 1st to high level in our 1e-adjacent games, though there's usually a few death-revival episodes along the way.You start getting meaningful power at around 7th level, but like I said, if you go straight at it you'll never make it IME. I can only assume that tales of players that have level up straight from 1st level as single classed Wizards or M-U's are either apocryphal or else they had much gentler GMs than I am or I'm used to.