I'm having trouble getting the FootIK controls to behave in a sane manner. I've restarted from scratch twice and, even though I'm not putting in any x-axis movement, it's in there and the feet get further and further apart as the rig moves forward. I tried it in 2.77a and 2.78c as well, but with the same results.
I should clarify:
When I say "as the rig moves forward" I'm talking about as it reacts to the cycle modifier (repeat with offset). With each iteration of the cycle, the feet move further apart. It's as if the modifier from the y-axis is also being applied to the x-axis (which has a Repeat Motion modifier, not a Repeat with Offset).
You've found something that I wish I could go back in time and change.
When modelers model a character, they like to turn the feet outward so it is more realistic.
But riggers like to have them pointing straight forward because it makes it so much easier to rig and animate.
I have rigged and animated both ways and don't really have an issue with it, but I should have insisted that the feet be straight to make it easier to animate for beginners. My bad for not seeing everything from the beginners eye.
That means you are encountering an issue earlier in your animation learning than I would like you to. But it is an issue that you are likely to encounter. So solving it is a pain in the butt but once you know what's going on, it should make sense.
The feet are pointed outward slightly, so as they get further away from the Root, their local y (I think it's Y) will get further away.
To compensate, you need to slide the x loc back inwards to counter this. Hope that helps.
(I think I will edit it when 2.8 is released)