Well, at least it was only a basic Dark Energy. They're rather common and easy to get now a days, honestly. I have an abundance of them after running my Mightyena deck in Nationals several years ago. I could send ya one to replace it!
Ahaha, that aside, this may be non-Pokemon but back in 5th grade, the only year I was ever interested in the Yu-Gi-Oh TCG. I purchased a starting deck with the Red-Eyes Black Dragon as it was themed after one of the characters (The guy that looks like Light Yagami). There should have only been One Black Dragon, but I got TWO holographic Red Eyes Black Dragon cards!
So TCG errors seem to happen more often than one may think perhaps. Thankfully they're easy to fix. Most miss-prints even in the Pokemon TCG get adjusted rules to accommodate their errors so they're not so much of a biggie these days in the actual TCG Tournaments.