I picked up a 1989-D to put it in the cull pile. Something didn't feel right, so I put it on the gram scale. This zinc Memorial should weigh 2.50 grams (+/- 0.100 g.) but the scale read 2.34 grams. That is 0.060 g. less than the 2.40 grams that should be the lightest weight. This coin has lived a rough life and deserved to be retired to the cull pile. If you had a 2.50 g. normal coin and removed the copper plating it should weigh 2.4375 g. This coin still almost all of its copper plating. It does feel thinner than it should as seen in the attached photo. I can't explain this, so if you have an answer, please post.
If the copper plating is exposed some of the inner zinc can an will errode away. This is what has occurred on the memorial building. Thus the lighter weight. Not a mint error of any kind.