My worst one was accidentally overwriting my backup when trying to clone it.
I was using a standalone drive cloning device and I mixed up the "source" and "target" slots. It was a 4tb drive so the operation took about 3 hours.
At the end, i plugged in the clone to check it and saw that it was blank. I ended up having to make a new backup before i was able to try the cloning again.
Since it was a backup, nothing of value was lost, but it sure was a waste of an afternoon