For starters, Edna's many, many responses when using items with, well, just about everything and everyone in the asylum, whether it's required for a puzzle or not. Harvey, Comic Book, and Police Badge are just the tip of the iceberg.
When she encounters an inmate in a bee suit she asks him why. He gives off a variety of complex reasons. Until Edna says...
Edna: Admit it, you lost a wager.
Beeman: I lost a wager.
When Edna tastes the Stinkydrink she starts sputter causing Harvey to worry, until she finally responds:
Edna: The natives have surrounded the fort, Colonel.
Doing anything with the toilet mirror will cause Edna and Harvey to speak backwards.
There's also a hose there which Harvey believes to be a snake out to eat him. Using items on it will cause Harvey to explain why it will not work as a weapon against it.
Peter. While everyone in the Asylum is hilarious to some extend and usually goes along with whatever Edna wants, Peter is simply so depressed that she literally gives up talking to him at one point since he depresses her as well.
After finally getting the Master Key and accessing the backyard, Edna can look into a couple of trash cans right outside the door. Both are filled completely with nothing but Master Keys.
A meta example in the leopard mode: the manual explains that there is one and that it will unlock new locations, give special abilities and much more, but that the way to unlock it is hidden. It was simply a Shout-Out to Maniac Mansion and there is no such mode, but not only some players claim to have found it, they even made fake screenshots of the location to support their claim, confusing plenty of players for quite a while.
Jan Müller-Michaelis: ...There had been speculations about our secret "leopard mode" which we mentioned in the manual, yet didn't include. It caused a lot of turmoil, similar to Maniac Mansion, some even claimed to have found it - occasionally with self-made evidence pictures (laughs)note ...Es wurde auch lange über unseren geheimen „Leoparden-Modus“ spekuliert, der im Handbuch erwähnt wird, aber nicht enthalten ist. Das hat im Internet für einigen Tumult gesorgt, weil, ähnlich wie bei Maniac Mansion, einige sich den Spaß erlaubt haben und behaupteten, sie hätten ihn gefunden – teilweise sogar mit selbst erstellten Beweisfotos. (lacht)