Comment by SOLAR_FIELDS

#5 is the big one. I left WHERE off an UPDATE on "prod" data once. Fortunately it wasn't mission critical data that I ended up wiping and I was able to recover it from a backup DB. I never did anything in SQL without wrapping it in a transaction again.

I will note that depending on your DB settings and systems, if you leave the transaction "hanging" without rolling back or committing, it can lock up that table in the DB for certain accessors. This is only for settings with high levels of isolation such as SERIALIZABLE, however. I believe if you're set to READ_UNCOMMITTED (this is SQL Server), you can happily leave as many hanging transactions as you want.