Eagle Error: old version of device set is not present in the new version of this device set.
Today I was attempting to paste parts of an Eagle schematic I found online into a new schematic, and I was getting the following error message:
Package variant PTH1 in the old version of device set RESISTOR is not present in the new version of this device set.
This is caused by a library in the original schematic having some different package definition than the library in the new schematic. The fix is easy: In the original schematic, go to Library > Update All. Then you can copy and paste without the error!