Here's what I think is happening with some people getting the 18 points (3 codes) and some not:

First, the 3-point code that's all letters is a cap code that would only have worked once for one person.

The three codes offering a total of 18 points were posted on FT and elsewhere several weeks ago. They seem to still be valid for anyone who has not already entered them into their account, but I bet those getting the "maximum number of redemptions exceeded" message are those who had already entered the codes into their account after they first appeared.

One other thing that may occur to someone (it occurred to me, after all): The "tell a friend" deal on the mcr site allows you to refer someone to sign up for a new MCR account, and then when they deposit their first 50 points, you get a 10 point referral bonus. All you need for a valid new "friend" is an email account that isn't yet registered. So why not use all your available email accounts or even create new ones, and have each one enter the 50 point promotion code above, and earn an additional 10 points for each time you do this? The reason why not is that the fine print specifies that promotional codes do not qualify for the 10-point bonus. I guess somebody at MCR thought of it, too.