Carb icing
"[Carburetor] Icing can be acute when the air is moist (70 percent or higher relative humidity) and the ambient temperature is between 2°C and 13°C (35°F and 55°F)."
If I recall correctly, the venturi effect is largely resonsible for the icing.
I hate it when this happens:
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001214X37841&key=1
