Music

Depending on the impedance of the headphones it could damage the device. Modern devices are generally more tolerant than old ones in that regard

In a solid state device, the lower the resistance the higher the power given a constant voltage (yes, there are voltage switching circuits based on resistance but I'm just doing the basics here) because P=V^2 /R. When you put resistance in parallel like this it lowers that bottom number.

So you could have far more power going through a circuit than it is rated for. Best case scenario you pop a fuse. Worst case there's a fire.

Relevant but boring story (I'm old, this is what I do): Back when I was cutting my teeth on electronics but before I had any formal education in it, I had a shitty little guitar practice amp. I didn't look at the impedance rating and replaced it with a pair of car speakers in a cabinet (I was just trying things! I was a kid!). Played through it for a few minutes and it was fine. Cranked it up and something inside popped.

Turns out it was expecting 16 ohms. The speakers I replaced it with were 2 speakers at 4 ohms in parallel, so 2 ohms total. Luckily I just removed the chassis and replaced the fuse. I learned a lot that day.

Edit: I have no idea how to fix my formula formatting. So I just added a space. Imagine there's no space, I guess.