I know there have been some threads about an archer stacked in a city and firing despite there already being another unit garrisoned in that city. This is a bit different in that the archer in question isn't even firing, just passing turns, stacked with a pikeman.
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I think you can have a garrisoned land unit, a land unit on the city tile that provides no bonus to the city, air units, cruise missiles, one naval unit, and a worker or settler or great person in a city tile. I think this is the maximum allowed.