There are several scenarios in which you could send Bitcoin to an “incorrect” address. “Incorrect” is the address you didn’t want to type in, whether someone uses it or not.
There are valid addresses and invalid addresses. If you’re lucky, you just made a typo and the typed address is simply invalid. If this happens, some part of the network (nodes/protocol/miners) won’t let you proceed with your transaction. This means that if you mistype, your transaction will probably be rejected and refunded.
Even though this can be helpful, be careful, there are many cryptocurrencies which may not have safety checks so you can lose your refund. The best solution is to always double check the address.
Another scenario can end up worse. If you send coins to an incorrect but valid address then your funds could be gone forever. Digital currency protocols are irreversible and transactions can’t be cancelled. Have in mind that every address exists on the network, only, most of them have 0 balance and there are no users that have a private key for it. If you send your coins to such an address, some wallet apps allow you to locate your funds and then try to refund the coins to you.
If you send coins to the wrong address and there is a user with its private key, and it happens that you know who it is, you can ask them to give your money back. This could happen if you type in an address you recently used.
The good news is, if you’re sending funds over email, after 30 days, any digital currency that has not been claimed by the recipient will be returned to the sender.
Finally, sometimes users accidentally send funds to their own previous address. Luckily, addresses can be used multiple times. If you used it in the past, it should be fine to use again.
The bottom line is, even though there are some solutions to this potential problem and funds theoretically could be returned, it is better to be safe than sorry, so keep your eyes open and be extra careful when you’re trying to send your funds.