Learn English – Word (or phrase) that means something you thought was a great idea, that turns out to be a massive error

Answering the general question, the word is proved. In fact one definition for prove over at both Wiktionary and Merriam-Webster is literally "to turn out to be". Quoting the former,

3. (copulative) To turn out to be.
Have an exit strategy should your calculations prove incorrect.

In your specific example sentence, however, I'd prefer actually saying "turned out to be". Otherwise the register seems off.

I think you are along the right lines. I would say that something like a switch can be toggled.

You can also say that it can be switched on or off. However, usually if you can switch something on, there is the case that you can also switch it off.

You may also have a context related word such as flicked for lights and powered for computers.