i have joomla 3.2.0 and i installed K2 v2.6.7 and created 3 field groups and add some extra fields inside them and assign them to some categories when i try to create a new item and choose the category i assign the field group too and hit save the the item doesn't save but when u try any other section with no custom inside it save correctly

Please update to Joomla! 3.2.1 and K2 v. 2.6.8 DEV build - getk2.org/assets/get/

Let me know if that solved the issue