Redmine

You can fix this one with a little bit cleaner code:

Change:
<button class="tab-left" onclick="moveTabLeft(this); return false;"></button>

to:
<button class="tab-left" type="button" onclick="moveTabLeft(this);"></button>

Browsers defaults often to type="submit" which submits the form. If you explicite define a type, it will be fine.
The return false; is not the cleanest way, as... if you call some code afterwards, it won't be executed because the propagation ends there.