Have you discovered any undocumented variable? Edited

[milynnus]

While working on a solution to show C when A<B using Butler for @Brett K_ on his question

I found out that there is a variable {cardlastactivitydate} . This is not documented in Trello Variable Help

and Yes, you can handle conditions with Butler - details see here

look for module in module list under Knowledge Base Page.

Have you come across other undocumented variables?

[Stephen Addis]

Only those created by users that we would put in the idiot classification, not allowed near a keyboard or what would otherwise be defined as "user error".

Work arounds are the only thing that we have as the genuine answer, but their not say variables but more ways to beat the system.

[milynnus]

@Stephen Addis 

In Singlish, your comment is “chim”. In any case, 🙏 for sharing. 
Looks like the variable was from Butler Bot. Found it here https://trello.com/c/MDLEnYg7/26-whats-new

Tested it just to be sure and found that it works.