That's not a text error at all - that's the program being snarky at you. There's no need to change this.

Eh, seems more along the lines of: "Your [insert item name here] is broken and thus useless."
If it is an error, it would be better to remove the first 'broken': "Your broken triangular key is broken and thus useless."

Overall it shouldn't say that because it's redundant, so the second post is what we would have to go with. lol

What about "Your broken triangular key is, well, broken and thus useless"?

Adding the "well" would ruin it, too. The program would now acknowledge that it's being snarkily obvious, rather than just being snarkily obvious. Makes it sound awkward rather than matter-of-factly.

There's also a good reason that the message is snarky in the first place - it's the result of the player using a key to open a locked door after failing to open it the usual way thanks to Auto_Open_Doors = true in the ADOM config. It's not one message, it's two, and the latter is the same one you would get if you intentionally tried to use a broken key to open the door. To fix this case of the program showing a sense of humor you would have to make it check whether a use key attempt of a broken key was the result of Auto_Open_Doors or not and then muck about with the message output so that it shows something different in either case.

It's not a truckload of work to accomplish that, I'm sure, but since there is nothing wrong or confusing about the message, let broken keys be broken, I say!