Oh dear... I once logged into the postgresql database of a very busy hosted service in order to manually reset a user's password. So I started to write the query:

UPDATE principals SET password='

Then I went and did all the stuff required to work out the correctly hashed and salted password format, then finally triumphantly pasted it in, followed by '; and newline.

FORGOT THE WHERE CLAUSE.

(Luckily, we had nightly backups as pg_dump files so I could just find the section full of "INSERT INTO principals..." lines and paste in a rename of the old table, the CREATE TABLE from the dump, and all the INSERT INTOs, and it was back in under a minute - short enough that anybody who got a login failure tried again and then it worked, as we didn't get any phonecalls). It was a most upsetting experience for me, however...