1. Start NetBeans with clear userdir & cachedir
2. Activate JavaSE, PHP, C/C++ modules.
3. Goto Tools >> Options >> Formatting >> Language: C
4. By default, Category will be "Formatting Style" and Style will be "NetBeans".
5. Check/Uncheck a checkbox and notice the Apply button works as expected.
6. Check all "Category" and the Apply button works as expected.
7. Change Style to Apache and Category to "Formatting Style".
8. Notice the Apply button works as expected.
9. Change Category to "Blank Lines" and notice Apply button works as expected.
Apply button works as expected for all Category under Style: Apache.
10. Change Style to GUNU and notice that Apply button does not work. Change to
any Category except "Formatting Style" and the Apply button does not work
11. Change Category to "Formatting Style" and the Apply button starts working. 
Now the Apply button works for all Category under GUNU.

Even if you change back to Style: NetBeans in step #10, the Apply Button will 
not be working although it worked in earlier steps. This sequence works to 
disable the Apply button the same between any working Style and another style.

This behavior occurs only for Language C, C++ and C/C++ Header.

Product Version: NetBeans IDE Dev (Build 201401240001)
Java: 1.8.0-ea; Java HotSpot(TM) 64-Bit Server VM 25.0-b65
Runtime: Java(TM) SE Runtime Environment 1.8.0-ea-b123
System: Windows 7 version 6.1 running on amd64; Cp1252; en_US (nb)