I couldn't find a topic on that, but there must be one!
One to gather all that small things. Strange, hidden or way too obvious to know. The ones that you find by accident in some small thread on or irc.
So write here them all. I guess small things from manual that get missed should be included too. Just small things though.
Examples:
- Elixir of Life works on Lifeless beings
- mages with only random picks can be built without a lab
- trampling does not give you gems from Sickle Whose Crop is Pain [someone could acutally test what can give them]
- Domes stack
- immobile units cannot use Teleport etc
I am sure that someone has much better examples
