CAS precondition mismatch - row is still inserted

Cassandra claims CAS precondition didn't match current values[1], but the row is still inserted.

We are having a quite simple cf schema:

create table index (
  u text PRIMARY KEY,
  abc set<uuid>,
) WITH
  compaction={'class': 'LeveledCompactionStrategy'};

CQL statement:

INSERT INTO index(u, abc) VALUES(?, ?) IF NOT EXISTS

Sometimes, especially during some write load, Cassandra claims the conditional insert wasn't satisfied ([applied] false), but the row is still inserted.

The response from the conditional insert contains [applied]=false + the data we tried to insert.

A full CQL trace is attached.

[1] CAS precondition CAS precondition org.apache.cassandra.cql3.statements.CQL3CasConditions@1b2576ce does not match current values ColumnFamily(username_index -

- [:false:0@1427889576526000,abc:39f1a470ee694761a8197c0d0ada8e8f:false:0@1427889576526000,]) | 11:59:37.060 | /10.0.0.1 | 533702