unwanted

Zemblanity is when something unlucky, unwanted or undesired happens by design because it's already built in.

And finally, a side effect is an unexpected and usually unwanted secondary effect of a drug or medicine.

'Waste' is things that are unwanted like rubbish, and we can also describe something as 'a waste' when someone uses something they don't need to, or they throw something away after not using it.