The sarcasm. It is lost.

After every phone-call I take, I need to register the call. If I solve the problem, I write down how I did it and a copy of this is sent to the caller. (We supposedly do this so that, when the problem occurs again, they will be able to solve it ourselves. They don't.) Now I always try to keep these solutions perfectly civil, no matter how incredibly stupid the other person was being. However, this doesn't mean I can always curb my sarcastic tendencies. This leads to things like this:

Earlier today we spoke over the phone because you couldn't change your password. 

The cause of this was that to change your password, instead of pressing 'change password', you pressed 'registration'. When you pressed 'Change password', your password was changed and you could log in.

The idiot is implied pretty heavily here, right? Right? Right?

And thus I got an email back from this costumer, thanking me and complimenting me on the fast service and clear instructions I'd given her.

Right.