sorry, forgot to write about it. I tested on Ubuntu 16.04, 10.2.10-MariaDB-debug, commit b94a62b5dfa48; --repeat=200

I got his today with latest 10.2.12:

CURRENT_TEST: connect.tbl_thread
— /usr/local/mariadb-10.2.12-linux-i686/mysql-test/plugin/connect/connect/r/tbl_thread.result 2018-01-02 13:30:39.000000000 +0000
+++ /usr/local/mariadb-10.2.12-linux-i686/mysql-test/plugin/connect/connect/r/tbl_thread.reject 2018-01-02 14:07:46.835927257 +0000
@@ -90,17 +90,17 @@
set connect_xtrace=1;
SELECT * FROM total order by a desc;
a b
-19 test19
-18 test18
-17 test17
-16 test16
15 test15
14 test14
13 test13
12 test12
11 test11
+11 test11
+10 test10
10 test10
9 test09
+9 test09
+8 test08
8 test08
7 test07
6 test06

This is in buildbot on kvm-bintar-trusty-x86 (note 32 bit)

When running "mysql-test-run --repeat=100 connect.tbl_thread" I got after a while:

CURRENT_TEST: connect.tbl_thread
mysqltest: At line 60: query 'SELECT * FROM total order by a desc' failed: 1296: Got error 174 '(1064) You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '' at line 1 [SELECT ]' from CONNECT

More information that I hope is helpful:

CURRENT_TEST: connect.tbl_thread
mysqltest: At line 60: query 'SELECT * FROM total order by a desc' failed: 1296: Got error 122 '(1064) You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '`, `b` FROM `rt3``, `b` FROM `rt2`' at line 1 [SELECT `S' from CONNECT

In the mysqld.1.

In var/log/connect.tbl_thread/mysqld.1/mysqld.log I found the following
that may explain the problem:

697 Query CREATE TABLE total (a int, b char(10))
ENGINE=CONNECT TABLE_TYPE=TBL TABLE_LIST='t1,t2,t3,t4,t5'
OPTION_LIST='thread=yes,port=16040'
697 Query set connect_xtrace=1
697 Query SELECT * FROM total order by a desc
701 Connect root@localhost as anonymous on test
701 Query SELECT `a`, `b` FROM `t1`
701 Quit
702 Connect root@localhost as anonymous on test
702 Query SELECT `SELECT `a`, `b` FROM `rt3``, `b` FROM `rt2`
702 Quit

The problem is probably the extra SELECT` that was added first in the query.

When the above test goes right, then it log file looks like:

4 Query CREATE TABLE total (a int, b char(10))
ENGINE=CONNECT TABLE_TYPE=TBL TABLE_LIST='t1,t2,t3,t4,t5'
OPTION_LIST='thread=yes,port=16040'
4 Query set connect_xtrace=1
4 Query SELECT * FROM total order by a desc
8 Connect root@localhost as anonymous on test
8 Query SELECT `a`, `b` FROM `t1`
8 Quit
9 Connect root@localhost as anonymous on test
9 Query SELECT `a`, `b` FROM `rt2`
9 Quit

So it's clear that the problem is the extra garbage in front the query or
alternative connect things that the name of the first column is
"SELECT `a`, `b` FROM `rt3`"

After looking at how the query is generated in TDBMYSQL::MakeSelect()
I am inclined to think that the problem is that the field name somehow
gets wrong if the table object is used many times.

While running tests in a loop, I also got an error at:

#4 0x0000000000bd09e6 in STRING::GetNext (this=0x7fb150001ad0)
at /my/maria-10.2/storage/connect/xobject.h:153
153

(gdb) p G
$1 = (PGLOBAL) 0x20606260202c6061
(gdb) p G->Sarea
Cannot access memory at address 0x20606260202c6061

This could also mean that you have a memory overrun somewhere

See MDEV-14214