I use AI Servo nearly all the time... And in order to do that, to have it "act like" One Shot in some situations, I have set up and learned to use Back Button Focusing. That makes a world of difference. I simply never use AF linked to the shutter release button any more... haven't used it in years. I only switch a camera off of BBF if I'm lending it to someone who has never used that technique and isn't familiar with it or seems reluctant to try it. That doesn't happen very often.
If using multiple AF points, you are leaving it up to the camera to decide the point of focus. It will usually choose the closest object that's covered by an AF point.
If you were using a single, pre-selected AF point, check your images in Zoombrowser or on your camera's LCD. I bet that you let that AF point slip off the subject and that's why you lost focus. "Focus and recompose" is a common One Shot technique, that can't be done in AI Servo... unless also using BBF. Once that's set up, you can start and stop AF at will, anytime you wish.
One thing you won't see with AI Servo is Focus Confirmation. That only works in One Shot mode. So you have to learn to really trust yourself and your camera.
Depending upon lens and ambient conditions, any focus mode can take a moment to achieve focus and lock onto the subject. Sometimes shooting too fast causes errors (it also can lead to camera shake, "stabbing" at the shutter release button).
Different types of lighting can fool AF, too. Fluorescent lamps, in particular, can cause problems (with metering too). It actually flickers on and off 60X a second, which our eyes don't see, but our cameras can. Stage lighting might be tricky, too.
AI Servo isn't perfect, either. It might miss occasionally, if you trip the shutter as it's working or if the subject changes direction, or just for mysterious reasons. Some cameras allow you to prioritize focus accuracy over shutter release, which might make for slower shooting, if you demand as accurate as possible focus. But I don't think the 60D has any settings of this sort. Practice using it helps, too.
I do switch occasionally to One Shot... when working with a stationary subject and wanting to insure accurate focus, such as shooting a setup that will produce really shallow depth of field, so wanting Focus Confirmation and all the other help I can get. If I'm really concerned, I'll switch to Live View because it gives the most accurate focus of all, though it's slow.
BBF works fine with other focus modes, too... it's a popular sports/action shooter technique, so probably is most often used with AI Servo... but it's no problem using it with One Shot too, so there's no need to turn it on an off. It becomes second nature to control AF with your thumb, in pretty short order.
Another suggestion... stop your lens down a bit. That creates more depth of field so that some minor focus error is hidden, that might have been a problem if using a lens wide open, with particularly shallow depth of field.
EDIT: I think what rrblint is refering to is that when using BBF, you can treat AI Servo as if it were One Shot. No, it doesn't actually change back and forth between the focus modes. Some of the newer cameras have an assignable button, with one option being to switch between AI Servo and One Shot. Even with cameras that don't have that convenience, you can learn to make the change while keeping your eye to the viewfinder. The more you can change all the camera settings like that, the faster you can adapt the camera to changing situatioins and keep shooting.
Another thing occured to me... Be sure you aren't accidentally using AI Focus. It's not really a focus mode at all... In AI Focus, the camera is supposed to decide whether the subject is moving or not, then switch to the appropriate mode: AI Servo or One Shot. Now, I haven't tried it in years... but the last time I experimented with it, it caused an unacceptible delay that made me miss focus on many more shots (and that was with one of the most sophisticated 45-point AF systems, no less!)
lol)!!
)...just my 2 cents..
