When I was relatively new in my first job I forgot to include the WHERE clause in an update, essentially resetting the value for the entire table. Needless to say I felt awful and I was ready to hand in my resignation right after the issue was sorted out (I even printed my resignation letter). Luckily there was a relatively recent backup (not as recent as it should've been though... but I obviously wasn't the DBA) and things went back to normal relatively soon. Throughout the process the team shared their DB-related war stories with me. Everyone seemed to have had a similar experience happen at some point during their careers and knowing that made me feel a lot better. I ended up changing my mind and decided not to quit.
Much like the folk putting echos into find and wildcarded shell commands to check the output, I'll often start manual sql updates by doing 'SELECT something FROM database WHERE...' and check the output rows match my expectations before hitting the up arrow to replace the SELECT with an UPDATE.
For bigger tables, I use COUNT(something) if I expect it to be long but I have an idea of rows affected, or LIMIT if that's going to give me an idea that it's doing the right thing.
