Mere frost on wings and flying surfaces are insidious. Iced wings are obvious. But when the wing is contaminated the critical angle of attack for stall is much lower. Airplanes big and small have stall warning systems that are designed for warning at a fixed angle of attack. When frosted or iced, wing stall is reached at a much lower value. The stall warning system is worthless under these circumstances. Deicing fluid on wings can also affect the critical angle of attack, but that is another story. On small airplanes only use Type I fluid.