Some bugs linger around for months and eventually get squashed and go away.

Others happen in the course of a few seconds, but cost you thousands of dollars or tons of embarrassment.

In my experience, the quick but incredibly painful bugs usually have to do with three areas. In the three areas below, one ill-conceived line of code can sink your ship for days.

Email

Sending thousands of e-mails only takes a few seconds. This means that you can massively screw up in under a minute. There’s no undo button after e-mails are in the wild.

Payments

Charging someone through your payment processor, like sending an e-mail, takes under a second. But, even more so than e-mail, you really don’t want to fuck that second up. If you charge someone incorrectly, or charge lots of people incorectly, then you’re probably going to be in the mud for a while.

Security

As twitter found out recently, the difference between <%=h and <%= is non-trivial. Forgetting just a single character in a view, or a few characters in a before filter can completely own you.

Definitly turn up your paranoia when dealing with e-mail, security or payements. If someone’s picture doesn’t upload occasionally due to an ill-conceived model, that not a huge problem. If 30k people get sent a bad e-mail, hacked, or overcharged due to three lines of code, that’s a serious bummer.