Ever since I got my prime lens AF-S 35mm f/18G I have got a couple of out of focus shots in portraits.
I have switched off my auto focus assist light when shooting low light photos of my kids. I find that the Auto focus is unable to focus at large apertures and does not snap the moment when I want it to.
In fact, if forced, it gives out of focus pictures. The best thing to do in low light with large apertures is to switch to manual focus.
Some things to watch out for when using a prime lens:
1. Takes a few seconds to focus compared to smaller aperture lenses. You can get poor snaps if you do not focus properly. Often a good thing to manual focus
2. If there are two subjects in 2 different planes (Example, people one behind the other), because of low depth of field at high apertures, you can get one of them blurred. Watch out.