Frost on a small portion of the coil is not unusual. Under certain conditions, ice may form on the evaporator coil. This is indicated by very cold output at very low air speed and the icing can be seen through the air inlet hole with the filter removed. If this should occur, inspect the filter and clean if dirty.
Make sure air vents are open and not obstructed. Units have a greater tendency to frost when the outside temperature is relatively low. This may be prevented by adjusting the thermostat control to a warmer setting.
Should frosting continue, operate on any FAN ONLY setting until the cooling coil is free of frost; then resume normal operation.
If frost condition persist, contact your local service provider for assistance.