Trying to create a new outfit with a name you already used
You've already got an outfit with that name.

Trying to equip the wrong familiar item
Only a specific familiar type ([Familiar name here]) can equip this item.

Trying to buy a second display case
You've already got a display case.

Attempting to change your name using the 'Change capitalization' screen
You can only change the capitalization, not the name.

Sometimes even the most obvious things have unexpected rewards....

Contributing a nonsensical amount of meat toward the maintenance of the Cannon Museum
That docent make any sense.

The crackpot looks at you strangely. Eh? What? Huh?
When trying to get more continuum transfunctioners.

Trying to add a second familiar
You've already got a familiar of that type.

Attack another clan without buying any grunts or Grrrlgoyles
Your Clan Army contains no soldiers, soldier. So just pretend you attacked instead of actually attacking, since that'd be exactly as effective.

Saving a custom outfit without assigning a name.
Your outfit must have a first name. Whether it's O S C A R or not is your business.

Trying to put more meat into your closet than you have

I'm going to assume you meant XXX,XXX.
You put XXX,XXX Meat in your closet.

Trying to /listen to the chat room you're already in:

You must listen to the channel you're talking in.

Responding to a trade without offering anything.
Empty offer. That's right. Empty.

Trying to trade the Hermit for more items than you have trinkets
You don't have enough stuff to trade for that many items.

Trying to trade the Hermit for an invalid number of items, entering a word in the quantity box, or not selecting an item to trade for
The Hermit looks at you with a confused gleam in his eye.

trying to go to the bugbear bakery when you aren't wearing the bugbear costume:

Begone from here, filthy human. Only Bugbears may shop here.

Any of the top three games in the casino when drunk

You're too drunk to gamble

Edit: Oop - just noticed a post earlier on saying the drunk ones don't count. Ah, well - next time!