In unknown circumstances, dragging a task card from a "Milestone" column to a normal board column may fail with an error like:
Duplicate entry '<projectPHID>-<columnPHID>-<objectPHID>' for key 'boardPHID'
I made a significant effort to reproduce this in T13074, but wasn't successful. It's possible adjacent changes have fixed it.
This is likely easy to fix if we can find a reproduction case, but we don't currently have one.