Presumably this has been going on as long as Cassandra has existed, but wanted to capture it here since it came up in an IRC discussion. This issue will probably show up on any cluster eventually.
1) Start with a 3-node cluster, RF=1
2) A 4th node is added to the cluster
3) Data is deleted on ranges belonging to 4th node
4) Wait for GC to clean up some tombstones on 4th node
4) 4th node removed from cluster
5) Deleted data will reappear since it was dormant on the original 3 nodes
This could definitely happen in many other situations where dormant data could exist such as inconsistencies that aren't resolved before range movement, but the case above seemed the most reasonable to propose as a real-world problem.
The cleanup operation can be used to get rid of the dormant data, but from my experience people don't run cleanup unless they're low on disk. It's definitely not a best practice for data integrity.