The 5D II is probably a fantastic camera compared to what was out 5 years earlier but it's focusing system is saddled with a lot of af sensors that are sensitive to one direction or another - horizontal or vertical.
As you can see from the image- the sensors have an orientation- and their sensitivity is perpendicular to that orientation. Get something that lacks contrast perpendicular to that- and in AI servo mode- it does not detect focus correctly.
In normal single shot AF mode- it'll probably be OK. But when you put such a camera in AI Servo mode, the camera assumes that the lens is in focus, or close to it, and releases the shutter when commanded. The problem is - if you were using anything other than the center point- well, if there is no contrast perpendicular to the sensor to aid the camera in determining if it is or not in focus- well, the camera accepts that it is in focus- and takes the picture.
This is why you want as many cross type sensors - if you use AI servo mode- because by their nature, it's a more accurate arrangement- more chances of you being able to achieve focus on the other AF points- because the orientation does not matter as much- if it can detect contrast- either vertically or horizontally- it attempts to focus.
Cameras like the t4i, 60D, 7D, 5DIII, have lots of cross type AF sensors. I believe most of the xxD line does as well. This helps these more pro bodies to work better in AI servo mode, and even to an extent in normal single shot AF mode..
Most oof shots that I've had were from lower models like the t1i- in AI servo mode. Like most others, going to more pro bodies - results in generally better performance in AI servo mode. 5DII would be an exception.. An experience owner of a 5D II would know that the single center point is the most accurate- and only real choice- and would likely use just the center point in AI servo mode. Or at least if you knew your camera, you would.
EOS 5D III, EOS 7D,EOS Rebel T4i, Canon 70-200 f/2.8 IS II, Canon 24-105L, Canon 18-135 IS STM, 1.4x TC III, 2.0x TC III, Σ 50mm f/1.4, Σ 17-50 OS, Σ 70-200 OS, Σ 50-500 OS, Σ 1.4x TC, Σ 2.0x TC, 580EXII(3), Canon SX-40, Canon S100
Fond memories: Rebel T1i, Canon 18-55 IS, Canon 55-250 IS, 18-135 IS (Given to a good home)...