The size of a tombstone is not properly accounted for in the memtable. A memtable which has only tombstones will never get flushed. It will grow until the JVM runs out of memory. The following program easily demonstrates the problem.
This program should run forever, but eventually Cassandra runs out of heap and craps out. You needn't wait for Cassandra to crash. If you run "nodetool cfstats test.test" while it is running, you'll see Memtable cell count grow, but Memtable data size will remain 0.
This issue was fixed once before. I received a patch for version 2.0.5 (I believe), which contained the fix, but the fix has apparently been lost, because it is clearly broken, and I don't see the fix in the change logs.