You may be using JCE as your editor. If this is the case, make sure you have the latest version installed.
Before updating JCE, I was receiving the following error (in error console) every time I clicked a tab:

Error: document.getElementById(c + ("_div_" + e)) is null
Source File: devo.mysite.com/administrator/components/com_k2/lib/simpletabs_1.3.packed.js
Line: 1