Some of our characters recently died (8 Shock Lizards in a surprise round) so we needed to build new 5th lvl characters. This is how I opened his introduction.
A rather large man walked in the inn wearing a Wizard's hat, robes that were two sizes too small and carrying a Spellbook and Quarterstaff.
Assuming I'm a wizard build, the party decides to run me through trials (along with the other character to join, a rogue) to see if he can make it in the party. I only had to pass one test. First came the test of strength. we were told to lift, then throw a log. Whoever gets farthest wins.
I stepped up when it was my turn and told the party "With my 19 Strength I easily pick up the log and toss." The party is flabbergasted, a wizard with 19 strength?
Next was the test of Magic. My character is to hit a can with a magic missile. He reached into a bag and muttered some words and a bright tiny sized object flew out of his hand. It missed of course. Magic missile can't hit Inanimate Objects, as I informed the party.
Next test was combat. I was to fight the Ranger in the party and if I could defeat him with a nonlethal Weapon I won. The ranger got the first hit in. I then told him "I grip the longsword in two hands, Activate Rage, then Power Attack." After the first hit, ranger gave up.
At this point everyone is really confused. I then explained my Characters Backstory.
He was from a barbarian clan that believed you gained the power of the enemies that you defeated in battle. He beat many barbarians in battle, making him an amazing barbarian. One day, He killed a wizard and took his name.
Now, Mervin Stormcrow the Wizard (actually a 5th lvl Barbarian) Has joined the party. Oh, the Magic Missile? Mervin carries a bag of rocks.
TLDR: Barbarian character Pretending to be a Wizard and the Party can't prove that he's not.