Klo, I don't understand what you are complaining about. Any transport system (Bus, train plane) will kick you off at the end of the route. If you are flying from New York to London do you expect to be able to stay on board after it lands in London? It could be going anywhere next (or back to the depot).
If you got on an stop No. 15 wanting to go to stop No. 3, but arrive at stop No. 20 then you got on the bus going in the wrong direction. Just get on one going the other way. You will probably have to pay again. Why didn't you do that when you got to the end of the route in NIce?
We have all got on a bus/métro going in the wrong direction at least once, part of the travel learning experience.
To address Caroline's original question, the official métro map is here: http://www.ratp.fr/informer/pdf/orienter/f_plan.php?fm=pdf&loc=reseaux&nompdf=metro
Bus map is here: http://www.ratp.fr/informer/pdf/orienter/f_plan.php?loc=reseaux&nompdf=bus_paris&fm=gif
Larger version with streets is here: http://www.ratp.fr/informer/pdf/orienter/f_plan.php?loc=reseaux&nompdf=bus_paris_geo&fm=gif
These are all you need, just print them out and take them with you, or pick up free copies at any métro station. You need to find out the métro nearest station to your hotel (this is as vital a piece of information as the street address), and the nearest station for your destination.
Forget about timetables, you no more need timetables than you do for lifts (elevators) in a building, just turn up and wait for the next one. And forget about Google maps.
Buses (and métro) run to and from all parts of Paris 7 days a week. Especially the very central Opéra area. Don't worry.
And, when you arrive at your first métro station, buy a "carnet" of tickets (10 tickets at a discount price). Same tickets are valid for bus and métro. You can change between métro lines on the same ticket, but not between métro and bus.