One day I was working that Chesham-Watford run (it used to be the arres end of a night turn) in the morning and a colleague just dashed on board at Ricky being late to get to Neasden for an early turn.... tee hee hee I thought as he got off at Croxley shouting expletives towards me.
There were (not sure if there still are) a few Circle workings that would go round to Barking or Hammersmith. For 'merriment' I would change the blind a little early so that I would be going West at Victoria with Barking (which was where the train was going) as a destination or Hammersmith going East. Such things are no longer allowed - no jolliment on the job these days.
Other one off workings were a Sunday night Picc reverser at West Kensington and a Bakerloo that would reverse at Kilburn High Road (though the KHR move was ECS).
There are a number of RR (Runs if Required or Rusty Railers) though you would need to consult the working timetables and as the RR indicates, they don't always necessarily operate.
I forgot they totally changed the Circle... and Hot & Cold ! - still think putting Barking up on the O/R at Victoria would be frowned upon though even if that is where the train is going.
There is also a booked Northwood shunt, southbound into the sidings and back out onto the north.
Which seldom seems to run, as illustrated on the occasion I insisted it did, and the SB-NB crossover duly failed behind me after I left the siding!