People are also unaware of the amount of damage that can be caused to a spamcan by a seemingly minor scrape.
Noth that the pilot of this
PA28 elected to fly it, unaware that he had damaged the wing spar attachments.
The wing is inherently strong and forms a long lever. If you apply pressure to one end of it, spread over an area, you are just as likely to cause serious and invisible damage to the root attachments with little or no visible surface damage at the point of impact if it is made of metal as if it is made of wood.
In the case of the PA28 above he hit a post. Localised pressure meant that surface damage was visible. If it had been something soft like a hay bale he would have caused just as much damage to the spar attachments but less visible surface damage because the pressure would have been applied over a larger area.
Mike