You know you've got the right wireless network name, password and authentication type. You know your Mac airport card is functioning properly on other networks, and you're well within wireless range of your wireless base station. Then why are you getting the dreaded "There was an error joining the airport network xxx" error message?

This seems to be a problem that crops up primarily for users that connect Mac computers to 3rd party WiFi access points, but it can happen with Apple Airport base stations as well. If you've already tried the ususal things, including restarting your WiFi access point, restarting your computer, removing any known wireless networks from your "Remembered networks" list in System Prefs, deleting memorized wireless network passwords from your Keychain, and running Keychain First Aid on your keychains (whew!), then you might want to try the following, which solved the problem for me:

Remove the wireless network from the problematic computer in "System Prefs > Network > Airport," remove keychain password items for the wireless network in question, and then turn the wireless radio off on that computer (choose "Turn Airport Off" from the Airport menu). Then, temporarily change the access point wireless security to none (i.e. no password required), and leave the network name (or SSID) the same. Then turn the radio back on on the client machine, and join the wireless network, which of course won't require a password. Then, change the wireless security back to whatever you were using (which of course was WPA or WPA2, since no-one in their right mind would use WEP, right??), and if you got the same results I did, the client machine will properly join the network with the proper name and password, and all should be well.