I am the proud owner of a brand new MacBook Air running Mac OS X 10.9.3 and after a few days I started to encounter WiFi connection problems. Long story short, the WiFi was disconnecting very often and I could not connect to my router unless I restarted the computer or after deleting all the settings from the System Preferences > Network.
After 2 days of searching for a clue/answer on Google I noticed that all these issues occur especially when I have a USB drive plugged in. So I started to investigate about this possibilty and it turns out that there is indeed an issue with Mac OS X 10.9.X when for example you have some device plugged in the USB port and the computer enters Sleep mode. In such cases when it is turned back on the Network interface behaves abnormally. (For example it says that the password for the WiFi network is incorrect or it simply doesn't connect to the network)