Well, my favourite would be "Tracking to and from an NDB using an RBI".
I never did this on any of my courses but had to teach it, so I worked out how to do so in the bath one night. At my next FI renewal, Frank Morgan (R.I.P.) asked me how I did just that. I got out the coloured chalks and spent much time explaining same on the board. "Capital", he announced " I had to work out how to do this in 1948 and invented the same system you use today"!
P.P.