Elided puzzle bug

An elided puzzle bug is a coding error in a work of IF, where the player character is able to claim the reward portion of a puzzle without solving it first. In short, the player can ignore the puzzle because the author didn't code it properly.

Variant 1: The non-obstructing barrier

Some puzzles reward players with a new location, but first block access to that location with some type of barrier, such as a locked door. Everything might suggest that the player can't go that way, but if the player can go that way anyway, it's not much of a puzzle, is it?

This example is from the initial release of The House (Owen Parish; 2003; TADS 2). Fortunately, this bug has since been fixed:

Passage
    The stairs from above are somewhat more battered in this room, which is extremely dusty. The floor is cold stone, and the door to the south is a particular sturdy looking wood, with three impressive locks.

>x door
It bears three impressive locks. It's closed.

>open door
It won't move.

>s
You walk through cautiously. You find yourself in [etc.]

Variant 2: The open vault

Some puzzles reward players with a new object. The object is visible but is either contained or guarded by something. Perhaps it's in a locked cage or a display case. Maybe it's held by an NPC who's hostile to you or has no reason to let you have the object yet. But maybe you can just take the object anyway?

Here's an example from Grounded (Justin Lado; 2006; Z-code). You're supposed to give something else to the brat to play with and then she'll drop the Coke. But nothing stops you from just taking the Coke regardless.

The bonanza of pink, home of the brat
Your sister is lying on her bed. 'Mom's still mad at you, don't talk to me.' How hospitable. The hallway is to the north.

You can see a brat here.

>x brat
Your younger sister, who is always swathed in pink. She's playing with a can of Coke and reading a book.

>take coke
Taken.

Variant 3: The invisible prize

In this variant, the prize object is not visible to the PC, but it is present and concealed from view. The PC is meant to do something that will result in getting that object. However, suppose the player guesses ahead of time that the object exists and just types "get [object]" everywhere? If the author solely relies on concealment to hide the object, and doesn't code anything to block a "take" action, then the player can obtain the puzzle's reward without solving the puzzle.

Here's another example from Grounded. The front door is locked, so the player knows that there's a key somewhere. The PC is supposed to get the key from dad, but not as easily as this:

The master suite
Your father is playing World of Warcraft on his computer in the corner. 'Aren't you grounded?' The living room is to the west.

You can see a dad here.

>x dad
He seems rather focused on World of Warcraft.

>take key
Taken.

[Your score has just gone up by one point.]