Verified on a Ubuntu 14.04.1, before the upgrade I can reproduce the issue easily with unity 7.2.3+14.04.20140826-0ubuntu1. Then I reset the settings and upgrade to unity 7.2.4+14.04.20141217-0ubuntu1, now the issue hasn't been reproduced yet after more than 50 log out&in. Thanks guys.
Verified on a Ubuntu 14.04.1, before the upgrade I can reproduce the issue easily with unity 7.2.3+14. 04.20140826- 0ubuntu1. Then I reset the settings and upgrade to unity 7.2.4+14. 04.20141217- 0ubuntu1, now the issue hasn't been reproduced yet after more than 50 log out&in. Thanks guys.
I'll update the tags now.