The few that jump to mind are:
Destroyed - Defeat the Destroyer for the first time.
Plantera's Furious - Defeat an enraged Plantera.
'Now do it with a Broken Wooden Hammer.' - Defeat a boss using only the Copper Shortsword.
Unification - Craft Night's Edge
True Unification - Craft Terra Blade
Haxxor - Be in possession of an item you're not normally supposed to have (unobtainable item, Dev armor) - This probably isn't a good one, as it will look bad on anyone who might accidentally pick up an item from others on a random server, but still is an idea.