with this little modification:
-  subcaps = gst_caps_from_string ("text/x-pango-markup; text/plain");
+  subcaps = gst_caps_from_string ("text/x-pango-markup; text/plain; subpicture/x-pgs");
the playbin-text.c example can be used as a testcase for this issue.

after the nth (in my case 6th) subtitle, playback stalls:

** Message: received a subtitle at position 0:19:31.755000000, running_time 0:19:31.756610000
00000000 (0x7f61c956d400): 16 00 0b 07 80 04 38 10 00 05 00 00 00 00 17 00  ......8.........
00000010 (0x7f61c956d410): 0a 01 00 03 50 03 63 00 e3 00 32 80 00 00        ....P.c...2...  
^C

* 1    Thread 0x7f61d5691700 (LWP 5202) "playbin-text" 0x00007f61d3d244ed in poll () from /usr/lib/libc.so.6
(gdb) bt full

+ Trace 235326

testfile available at http://dreambox.guru/test_stuck.mkv.xz