The card has seen its fair share of wording changes, and even had its HP changed from 10 to 50 for the EX Legend Maker print for no apparent reason.
The EX Sandstorm print was reworded to read as follows:
Template:Trainer Text
The EX Legend Maker print made an additional reword, changing "instead of a Trainer card" to "as well as a Trainer card"; this kept the card in line with a prior errata.
It was once believed that the card would have been reworded again in its next printing after the EX Power Keepers print to be similar to the wording on Armor Fossil and Skull Fossil. However, Majestic Dawn instead decided to do away with Mysterious Fossil and instead introduce Helix Fossil, Dome Fossil, and Old Amber as separate cards, essentially ensuring that there won't be another print of Mysterious Fossil.
Trivia
The Dome Fossil and Helix Fossil appear in this card. However, the artwork depicts the Dome Fossil as the fossil of a Kabutops rather than a Kabuto.
