An UPDATE sql statement with a forgotten WHERE clause
Same with rm -Rf /foo/bar. There's a moment when pressing enter will do the wrong thing. I usually use relative paths to avoid this, but I have had to repair the damage for someone who actually managed to rm -Rf / by accident due to this.
Also for Update/Delete query I run the equivalent select (rows or count(*) ) query first, just to make sure.
also make sure to write where predicate before completing the whole sql. (The sql does not go through by accidentally hitting enter, before the whole statement is written)
tl;dr - Run read operation before doing write/update/delete operations.