TEST:
On Zabbix web page :

• Log as super admin
• Configuration > Hosts
• Select Templates on the right
• Select many templates (4 or more)
• Click on "Delete selected"

Zabbix web page return an error:
"Fatal error: Allowed memory size of 16777216 bytes exhausted (tried to allocate 24 bytes) in /usr/share/zabbix/include/db.inc.php on line 519"

• Back on last page using the back button of the browser, displays the templates.
• Select only 1 template
• Click on "Delete selected"

After several minutes : Firefox display an error : 500 Server Error
When trying to display Zabbix web page on another firefox tab, I also had this error :
Zabbix page is displayed but there is an error message :
" * Error in query [UPDATE sessions SET lastaccess=1269512352 WHERE sessionid='7118b198f7ecd1a50afec1a0e7c5912e'] [Lock wait timeout exceeded; try restarting transaction]"

MySQL restart (/etc/init.d/mysql restart) solve the problem.

I reproduced this bug each time I tried.