Yes, it happens.
You typed in the wrong address, or you selected the wrong one out of a drop-down menu, or you sent funds or tokens to a contract address instead of a wallet address. Any number of things can happen, but if you're reading this article, you probably suspect that you sent funds to the wrong address.
What can I do about it?
Well, if you realize it in time, while the transaction is still pending, you can cancel the transaction. But again, if you're here reading this article, that time has probably already passed.
Check your transaction history on Etherscan if you're at all unsure of where you sent it, or where the funds ended up. If you know the party you sent it to, you could reach out to them and see if they would be willing to send it back to you.
What if I sent tokens to a contract address?
Unfortunately, there is no guarantee anyone can retrieve your tokens if you sent them to a contract address. A contract address is not a repository of tokens; it is essentially a computer program, a set of code, that performs some function on the blockchain. Some of them--the token contract addresses explained here--are addresses that can receive tokens. In this case, you may be able to reach out to the team that makes the token or manages the project, and they may be able to get your tokens back to you. However, some projects don't have customer support, which is another feature of the decentralized web.
Why can't MetaMask just reverse the transaction, or refund me the value?
MetaMask is not a bank, and it is not a custodial wallet--in other words, we do not keep any of your data. This is an essential security feature built into the nature of blockchain technology. You are the custodian of your funds. It is for this reason that we issue the constant reminder: Never share your Seed Phrase / Secret Recovery Phrase with anyone. If you do, that person has full access to, and control over, all your accounts and tokens.
To be clear: transactions are NOT reversible once final. It is NOT possible to "undo" or "reverse" a transaction once it is confirmed by the network.
What if someone from MetaMask DMs me and asks me for access to my account to help me?
MetaMask will *never* DM you, and will never ask for your Seed Phrase / Secret Recovery Phrase. If someone does this, they are most likely a scammer and should be ignored.
If you do have additional questions, seek assistance only through official MetaMask channels.