Is saving ice cubes from a finished drink a bad idea?

It would have some bacteria from your mouth, so it would be OK for you, but not so good for someone else.

I would not put it back with the general population of other ice cubes. @RocketGuy what if I used a reusable straw?

Well, I would.

It’s your ice, it’s your drink, if you want to, you can.

Don’t use it in other people’s drinks.

I think of ice as being disposable. I have an ice maker. If I purchase ice, it’s so cheap I don’t think it’s worthy of reusing.

If you keep it for yourself that would be fine. I don’t think people will appreciate having used ice in their drinks!!

If you keep it in your own cup that’s fine.

Please don’t put it back in with the fresh ice. Your backwash is in there, very unsanitary, and just plain gross.

I will put fewer ice cubes in the glass next time.

Agree with @Gideon2017

I’d just chuck it in the dog bowl, personally.

Not going to happen in my house.

it’s okay for you but not okay for others. Unless they know that it’s a used one.