It only happened to me once: I joined a game where I was given an established magic-user, and told nothing (this was 1st edition AD&D), and was annoyed by the way the GM made life difficult for me. I eventually died 1/3 of the way through when I cast lightning bolt at an opponent who had a ring of spell turning: the DM ruled, by fiat, that the spell was automatically turned and that I was instantly dead.
I got my (petty) revenge when I later started up a group chant of "The DM is a cheat!" among the players. Not one player held back....
Perhaps not exactly the same, but I have had this done to me. I left the game that night.
As a GM I would never force a pre-gen character/NPC onto a new player. I always give them an option: "Create a whole new character and we will work him/her into the game. However, if you would like to play one of these NPCs, here are the sheets I have for them."
I then let the player decide. If the player decides to go with a NPC, I allow the player to study the sheets as thoroughly as the player desires. I also answer any questions the player may ask about things I don't have listed on the sheets. I never rush them into the game. If necessary, we will end the playing session, and usually all persons help the new player with creating a new character.
Sometimes, the player will play a NPC until they get an idea for a character of their own. Then we end playing to help in the chargen.
As a rule, due to the bad incidences that have occurred in the past, I would rather sit and watch the role playing while also reading the GM's handouts for House Rules, CharGen, setting, history, etc. Then I will generally work to create a char that fits, tweaking as necessary.
rmfr