I'm one of those people that is encountering a problem with the wifi connection not being saved on the settings, thus always asks for password everytime you connect. Aside from always entering password, when you also open the wifi connection properties , you will notice that the "forget this network" button is missing. This is another proof that you got the error too.
This usually happens if you restore a previous back up after the iOS 6 update and the network settings are not properly restored.
If ever you have encountered this problem, here's two way to fix it:
1. Simply change the ssid of the router to a new name that is different from the current one
2. Change the wifi password (but rarely works) , just do number 1 :p