The old problem was:

* The el probe overwrote two bytes in the SMC card's EPROM area.

* The wt probe toggled the (undocumented) bit that told the SMC card
  to write back the data to the EPROM.

This corrupted the EPROM, causing the checksum to fail and the MAC
address to be incorrect, thereby causing even the SMC utility to be
unable to find the card.  This in fact caused a few people to think
their cards were completely broken and return them.