They can be hard to detect unless you use a strong backlight (a sunny window works well). One small knick will induce instability at a given rotational speed too. And if you replace any blades, insist on using genuine Parrot props - aftermarket props could possibly worsen the effect especially if you mix & match. Personally, if I locate as many as two cracks across the craft I replace every prop with new, and hand pick the best used props for 'emergency spares'. Pricey yes, but cheaper than a crashed craftIf you have not had a crash previously I would check your propellers for hairline cracks.