I think I've got them to appear pretty reliably.. so, steps to reproduce:
1) Have a city in an age below PE (or have no two-lane roads in a city)
2) Have a friend that has two-lane roads somewhrere, preferably in places you don't even have expansions, so that the incid3nts will always be visible. Not always possible, but you can build such a city with dev tools.
3) Play for one day normally, but not visit ANY city directly. Or watch after you have visited someone
4) Visit that friend
5) Check all places where he had roads for incidents.
More often than not you will get at least one pothole after this sequence. I expect the code that generates incidents creates incidents of all types for everyone, regardless of their age, then tries to place them somewhere in a valid location. Since potholes have none before PE, they don't get placed as a result (effectively reducing the number of available incidents for low-era players), but they get triggered to place some time after you return from visiting others, and use their layout to determine valid positions to appear. If the friend just visited had two-lane roads, the potholes finally get a place to appear, and you see them in weird places, as they are not aligned to your city.