There is a (known) race condition during counter read. Indeed, for standard
column family there is a small time during which a memtable is both active and
pending flush and similarly a small time during which a 'memtable' is both
pending flush and an active sstable. For counters that would imply sometime
reconciling twice during a read the same counterColumn and thus over-counting.
Current code changes this slightly by trading the possibility to count twice a
given counterColumn by the possibility to miss a counterColumn. Thus it trades
over-counts for under-counts.
But this is no fix and there is no hope to offer clients any kind of guarantee
on reads unless we fix this.