Hi there
If you must put previously used rough wood through a planer/ thicknesser or any other machine invest in a small handheld metal detector and spend time removing unwanted metal before machining it.
Good thinking ...very good!!...Before reading your comment I was about to suggest a strong magnet...which at least can give indication of ferrous metals.
Much of the wonderful old timbers are today just smashed up by demolishers...who are far more expensive per hour than dentists...when they could be de-nailed and used for other tasks than furniture. In Australia where cedar forests were cut down to extinction scores of years ago and used for many purposes (e.g. furniture and skirting boards) the old 'red rattler' and other early train carriages are a great source of it.
Some excellent timber can be found and reused from old solid timber kitchen cupboards bookcases, bedheads and the like. Metal-ware is easily located in them.
I winced at the idea that a chap doesn't care if his Ryobi planer gets smashed up...I have always considered a broken tool...or a broken drill... to reflect poorly on the user. It can of course come from an anxious user who has ADHD or Aspergers or is alcoholic...rip tear and bust rather than 'thinking it through'.
Big problems are not always 'Swiss cheese' alignment but from a number of small events in handling which lead to sudden catastrophe. When one doesn't care about the small damage, it does grow into missing the symptoms of 'trouble on the way'...Extreme examples are two boys in separate incidents with timber shredders....One fell into a large machine near Sydney Town Hall, decades ago leaning too far, more recently another was dragged through when his arm was caught with sapling he was tying to force through.
Decades ago developers on a building near our construction site at Surrey Hills introduced a dozer for clearing the floors. The floor collapsed under it and it went smashing through , as I recall, 7 floors killing workers as it went. The people did not'think through'their task and very possibly/likely successfully took other risks on other occasions.
The professional thing to do, as I see it, is to thoroughly examine timber for 'shrapnel' nails, screws, projectiles and the like before using it. Such potential destructors can be found occasionally in newly cut timber as well as used.