I can confirm the issue on PXC 5.5.34 + Galera 2.8 and PXC 5.5.37 + Galera 2.10.

This is how I tested (thanks to Celest for test case):

- created 3 nodes PXC cluster using Vagrant and these wonderful pack: https://github.com/lefred/percona-cluster
- installed parallel-ssh on vagrant host 
- created sample table:

CREATE TABLE `tbl_0` (
  `reg_name` varchar(127) NOT NULL,
  `reg_key` varchar(64) NOT NULL DEFAULT '',
  `reg_secret` varchar(128) NOT NULL,
  `description` varchar(255) DEFAULT NULL,
  `reg_status` varchar(16) NOT NULL,
  `user_id` varchar(255) NOT NULL,
  `owner_id` varchar(64) DEFAULT NULL,
  PRIMARY KEY (`reg_key`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

- enabled debugging on one of the nodes:
SET GLOBAL wsrep_log_conflicts="ON";
SET GLOBAL wsrep_debug="ON";

- deployed attached bash script on each node
- ran it in parallel on each node with:

parallel-ssh -P -h cluster1.pssh -x "-i /home/przemyslaw.malkowski/.vagrant.d/insecure_private_key -oStrictHostKeyChecking=no" '/home/vagrant/test.sh 20 100 1'
[1] 08:43:10 [SUCCESS] vagrant@127.0.0.1:2211
[2] 08:43:11 [SUCCESS] vagrant@127.0.0.1:2205
[3] 08:43:12 [SUCCESS] vagrant@127.0.0.1:2204

percona3 mysql> select count(*) total from tbl_0; select count(*) node1 from tbl_0 where reg_key like 'percona1%'; select count(*) node2 from tbl_0 where reg_key like 'percona2%'; select count(*) node3 from tbl_0 where reg_key like 'percona3%';
+-------+
| total |
+-------+
|   178 |
+-------+
1 row in set (0.00 sec)

+-------+
| node1 |
+-------+
|    43 |
+-------+
1 row in set (0.00 sec)

+-------+
| node2 |
+-------+
|    63 |
+-------+
1 row in set (0.00 sec)

+-------+
| node3 |
+-------+
|    72 |
+-------+
1 row in set (0.00 sec)

Debug info example:

140505 14:43:11 [Note] WSREP: trx conflict for key 0 74657374 74626c5f30 010050004500520043004f004e004100 : source: 5a67fd6a-cd2c-11e3-9ec4-de1070c656ba version: 2 local: 1 state: CERTIFYING flags: 1 conn_id: 7900 trx_id: 38507 seqnos (l: 24976, g: 24850, s: 24848, d: 24429, ts: 1399293791018546985) <--X--> source: d067275c-d44d-11e3-b054-036e4c0d59ea version: 2 local: 0 state: COMMITTED flags: 1 conn_id: 186 trx_id: 31963 seqnos (l: 24975, g: 24849, s: 24848, d: 24847, ts: 1399293791095596655)
140505 14:43:11 [Note] WSREP: commit failed for reason: 3
140505 14:43:11 [Note] WSREP: conflict state: 0
140505 14:43:11 [Note] WSREP: cluster conflict due to certification failure for threads:
140505 14:43:11 [Note] WSREP: Victim thread:
   THD: 7900, mode: local, state: executing, conflict: cert failure, seqno: 24850
   SQL: INSERT INTO
        tbl_0 (reg_name
                , reg_key
                , reg_secret
                , description
                , reg_status
                , user_id
                , owner_id)
        VALUES  ("percona1_name_75"
                ,"percona1_key_75"
                ,"percona1_pw_75"
                ,"percona1_descr_75"
                ,"active"
                ,"uid_dbs_75"
                ,"A_accessorLoadtestCk_1")
140505 14:43:11 [Note] WSREP: cert failure, thd: 7900 is_AC: 1, retry: 1 - 1 SQL: INSERT INTO
        tbl_0 (reg_name
                , reg_key
                , reg_secret
                , description
                , reg_status
                , user_id
                , owner_id)
        VALUES  ("percona1_name_75"
                ,"percona1_key_75"
                ,"percona1_pw_75"
                ,"percona1_descr_75"
                ,"active"
                ,"uid_dbs_75"
                ,"A_accessorLoadtestCk_1")
140505 14:43:11 [Note] WSREP: releasing retry_query: conf 0 sent 0 kill 0  errno 1213 SQL INSERT INTO
        tbl_0 (reg_name
                , reg_key
                , reg_secret
                , description
                , reg_status
                , user_id
                , owner_id)
        VALUES  ("percona1_name_75"
                ,"percona1_key_75"
                ,"percona1_pw_75"
                ,"percona1_descr_75"
                ,"active"
                ,"uid_dbs_75"
                ,"A_accessorLoadtestCk_1")