So I just got a Cateye Urban Wireless computer and took it for a first ride today. Everything was working alright, but then I intentionally take a steep downhill to test the max speed. It recorded the maximum possible number the computer displays, 65.9 mph. I've read about the unreliability of wireless due to interference, but this is my first experience with a wireless.

Is this something that will likely happen frequently, or just an unlikely occurrence ?

Also, is there a reason other than interference for which this could have happend ? (I have the sensor close to the rim on the spokes, if it makes a difference)

The interference is with extra radio signals being picked-up by the head unit (it has nothing to do with the transmitter). Basically, the head unit registers extra clicks (looking like more wheel rotations).

It's a characteristic issue with analog wireless computers (the digital wireless computers don't have this problem). Note that the erroneous max speed is not related to you going fast (it could happen even standing still).

It's relatively rare (you might see it 1 or 2 times out of every 10 trips).

You might have a problem with the position of the magnet with the sensor but that would show up as a loss of counts (that is, you would not see a high maximum speed if the magnet position was off).

(You really should lose the rude overly large sig image.)