Rotation order. Select your bone, go into the motion tab and change "axis order" to xzy. This will most likely screw up existing animation, so it's always best to set it before keying things. Keep in mind that different rotation orders work best for different situations. To expand on that a bit, it'll depend on what axis…
In a recent class I had at school I had to animate a falling scaffolding. I'm not planning on being an animator so I haven't done much outside of what I've had to do for classes and I ran into an issue where if I rotated a scaffolding sections "skeleton" from the first bone on only one axis, that it would not rotate on…
Another quirk that I just noticed, whenever I animate a bone, the shoulder for example, and Auto Key applies a new key, it applies the key for every bone in the limb, both lower and higher. Rotating at the shoulder sets keys on all the bones down the arm, as well as the clavicle above it. Rotating a food sets keys in the…