Forgetting to place a city on the board, or to move the robber. Which fix is acceptable?
1) The player paid for the city, they should get the city.  Now, if the next die roll would have given them double resources, that's where it'd get a little dicey (see what I did there?).  If they have the choice of where to put the city and want to put it where you get double resources, I'd say it'd be reasonable to say they don't get double that time.  If they only had one city choice, give them double.

If you can divine intent, give them the benefit, otherwise just try to smooth things out.

2) This is a weird miss.  You've had 2 turns of production (and potential non-production).  At that point, backing up would be a bit of a pain.  The easiest fix would be to immediately have the 7 roller move the robber, steal a resource, then proceed. 

This does not penalize the double-development-card player, since they might not have been able to even buy them if the robber was moved. This restores the resource parity, since the 7 roller didn't get a resource.