xM1 Proxmark ATQA:00 Anticollision Error

[Jirvin]

Not the one I originally recall reading but still a good source.
Also found this but still not the one I originally recall reading.

"magic" Mifare chips

The Mifare “classic” S50 1k chip? For many years the Mifare MF1ICS50 1k chip was used for all kinds of applications as a “secure chip” for everything from access control to stored value cards, and used for making localized payments within closed systems like public transit and laundry services. However, it uses a security mechanism called “crypto1” which is a simple, proprietary encryption mechanism that hardly has anything to do with modern cryptography. It has been broken for many years now, b…

More specifically this from that post:

A Mifare “magic” chip is a special grey-market chip made in China that can emulate the memory structure and functionality of real Mifare chips, but also allow sector 0 to be modified.