My cheap wireless bike computer is affected by one of my LED headlights (the NiteRider Lumina Micro 500), so I either separate the computer and light another inch or so, or use a Light & Motion Urban 500 that doesn't interfere with the wireless computer.

Other glitches with wired and wireless bike computers are usually due to alignment of sensor and magnet -- often there's a very narrow range in which the sensor will work, and it's not always by roughly aligning the magnet to cross the middle of the sensor module. Occasionally on really rough roads the wheel magnet on my bike will shift just enough to lose contact with the sensor.

With some unsealed wireless and wired computers sometimes water from rain, puddles or sweat can leak into the devices and cause shorts or corrosion. My cheap wireless computer has no weather sealing in the battery compartments for the magnet sensor or display. I avoid riding in rain so it hasn't been a problem. But I wouldn't expect it to function reliably if I did routinely ride in wet weather.