Got into a group for questing in Redridge on my first ever character. I was a rogue. I was used to playing RS, where I could switch between melee/ranged/magic at my leisure (and also used to using armor more as looks/prestige than actual usefulness).
Long story short, I thought that it would be more useful for me to chuck throwing knives at the enemy while my ally tanked all the damage. My friend, who had played longer and was watching this tragedy, quickly informed me that "that's not how groups work, idiot".
Also, playing sniper with that same rogue in Deadmines. Found a nice spot on the ship that unaccessible to NPCs where I could shoot them to my heart's content, and they couldn't do anything nor would they evade. This worked beautifully AFTER Van Cleef was already dead, but my groupmates would often (and justifiably) get mad if I started doing this during normal trash pulls. I believe this was before I figured out how to turn on battle text, so I'm sure I thought I was contributing a lot more than I actually was (cuz, cmon, I was shooting them with a gun, right?!?!)