Anyway IYKYK means If You Know You Know. It's often associated with something in the hobby that is known to hobby collectors, but the common collectors may not be aware.
I recently purchased a card off eBay for under $5 that tends to sell for over $100. It shouldn't be valued that much, but the urban myths surrounding this card make it very very rare despite being serial numbered on back to 2013.
Unlike many cryptids like Bigfoot and the Loch Ness Monster, this card does exist and has been documented with proof of it's existence. That doesn't make it any less rare.
There were some posts on blowoutcards back in 2013 and 2014 about this rare card which you can visit here.
Have you ever told someone that a card you wanted to buy was way underpriced?