What could be causing my MacBook Air 13inch (Mid 2012) 2 GHz Intel Core i7 OS X 10.8.4 to throw the "could not join" WiFi error?
I have done some in depth analysis for the same error on my MBA.

Basically there seems to be a MBA software problem somewhere in the Airport application.

In my case it looses the last digit of the Network Name (SSID) and then it claims it can not find it (what a surprise).

Then it issues the generic Connection time out Error.

Here is what it looks in my Console:



The actual name is Eurona900850808-81
as shown in the WiFi scan.

I would check the Console report on your computer to see what is going on.

If you need help how to get the report on your computer let me know.

I have been able to partially overcome this issue by disabling the Auto Join, and Auto remember networks.