I've recently noticed a number of weird channels that pop up from time to time in airodump-ng. These access points include various channel numbers like 108 or 133, have their rate set to -1, and come with an empty SSID (<length: 0>). I've seen these pop up many times in multiple areas, and I've seen them in multiple versions of aircrack-ng - first tested with the version that comes prepacked with BT4b and later with an updated version of 1.0 rc3 r1508. I'm not sure whether this is a bug in airodump-ng or the drivers, but I thought it'd be worth reporting. I've used my Alfa AWUS036H to make the capture.
Here is a screenshot (check the last two) and a capture file.