I have recently been trying to build a 2-part Opt-In using a Transactional template and had a very hard time getting the Variables correct, until a support operative corrected my syntax.

When adding variables and syntax is used, {{var:name}}, when not supplying a Default value (such as in a URL) the Variables list in the Example Code presented when the Template is Save/Published does not contain the {{var:name}} variables specified in the Template. Furthermore if you use incorrect syntax such as {{var:name:}} then they do appear even though they aren't valid!

The combination of the above issues lead me to waste a lot of time trying to understand why, when the variables I was using seemed to be valid the Template I was working on failed. It's extremely confusing!