Well, put it this way. Obviously none of us would deliberately fly in known icing conditions, in an aircraft not approved for such flight. We are far too wise and sensible to want layers of ice building up on our flying surfaces and prop. However, some people may have occasionally been in the following position.There may be a thin layer of cloud well below the freezing level. You may be cruising at 10,000 feet and have your airframe thoroughly cold-soaked. When you descend through that thin (but warm) cloud layer, the residual coldness of the airframe may lead to very short-term icing. If your wing ices up you will see it happening, it will start as light icing and get heavier at a certain rate, then it will melt and recede. The effect is proportionate and you can probably see what's going on. Pitot icing may exhibit itself in a sudden and apparently counter-intuitive way, as instrument readings go from sensible to haywire in seconds. Pitot heat will prevent this happening.