If you were just trying to modify source/dev with forge
1.download forge source
2.run install.bat/py/sh
3.wait for a long time
4.open the mcp folder in the forge source folder.
5.do what you usually would.
hope this helps.
It also happens with IC2 chargepads mod, and basically anything that modifies metadata while the item is held. Hopefully the forge devs will find a way to fix this soon.