The one that springs to mind for me was a quick and dirty update against a single entry in the database.. check the SQL, run the code, 1000000+ entries updated.
I had the where clause on its own line and somehow had the rest highlighted when i hit F5 to run it.
The panic and 'Oh nooooooooo!' was horrifying.
Luckily I had my transaction log backups in place and was able to get all but about the most recent 5 minutes worth of data back in quickly, and very thankfully before most of the end users had logged in for the day. Make a habit of checking your backups!
Sounds VERY familiar. Lost count of the number of "heart dropping through the floor" moments I've had over the years doing something similar :-D
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