You know how it is. The preacher spends 10 minutes explaining how we all matter - in God's Kingdom there are no red-shirts. Only gold ones. The congregation of elderly women and their grandchildren look at the (male, 55-year-old) preacher in blank incomprehension. The preacher wraps up by saying "and, after all, we must all be like children in God's Kingdom," sets photon torpedoes to full power and mentally takes out the back row, says the closing prayer, puts on his anorak and wanders out into the cold.
The illustration that nobody gets. Either it's too complicated, or it's a subject only the preacher could understand. Or it's aimed at the wrong people. Or even the preacher doesn't understand either the illustration, or the thing that is to be illustrated. So here are some examples of which areas of illustration not to use - and when.
Anything that itself needs explaining.
Army metaphors for the Christian Life that reveal your hidden desire to attack Belgium.
Astronomy except to say "Isn't space like really big?"
Contract Bridge - especially that tricky occasion you made 2 over on 3NT but everyone else went one down going for the slam.
Double Entry book keeping as an illustration for sin and redemption (or anything else)
Entropy - See "Quantum mechanics."
Fine Details of railway timetabling.
Giles Fraser's articles on the Guardian website. Not as well-known outside Social Media as you think.
... is a bit like the Trinity
Keith Lemon - unless the entire congregation consists of TV programming commissioners, nobody will have watched him.
Luxury Foreign Holidays
Monty Python's Life of Brian
Mumford and Sons lyrics except at "Hipster Church".
People in or known to the congregation - without prior agreement. "You know, Angela was telling me what it's like suffering from piles, and I'm sure she won't mind me telling you..."
Richard Dawkins - see Giles Fraser
Quantum mechanics - unless the congregation is made up of physicists or chemists. Nobody will understand it.
Quantum mechanics - if the congregation is made up of physicists or chemists. As it will be discovered that actually the preacher doesn't understand it.
Reminiscences of Margaret Thatcher.
Science Fiction or Fantasy literature, unless at "Geek Church"
Things that show how clever the preacher is - as nobody will believe it anyway.
What it's like to be a bishop