Unless it's a real lot of money going through the courts to get reimbursed would be impractical and if you transposed two numbers while typing an account number then you might not even know who the money went to.
No, the law gets involved and at some point, a judge will decide whether you owe the sender the amount they mistakenly sent. It's basically the same for "I mistakenly sent you 100 that I meant to send to my friend" and "I mistakenly sent you 100 when I only meant to send you 50 that I owe".
And yeah, it's a hassle. At least you can generally find out which account the money went to, and with new IBANs, you need to mess up multiple digits, since they include a checksum. If the checksum doesn't match, the money bounces back into your account. Previously, some banks required that the account number and account holder match (within reasonable limits, misspelling the last name would work), but that changed with the IBAN system, only the account number matters now.
There's a good reason for that: nobody want's banks to take money out of accounts because they "feel that's the right thing to do".
There's one notable exception: SEPA direct debits getting reversed. Using DD requires a special agreement with your bank, however, and it won't happen that money ends up in your account without you requesting it - or if it does, you will get a phone call from your bank where somebody explains the situation, apologizes a dozen times and asks you to look into the matter and authorize a reversal. Acting on their own would be a criminal matter and likely be of interest to the regulators as well.