Swinging the leg without first taking the angle.
Why it failsYour hips push them backward instead of rotating them sideways and nothing tips.
FixShrimp out to the side so you are perpendicular before you swing.
Leaving their posting arm free on the sweep side.
Why it failsThey post, stop the rotation, and you have opened your guard for nothing.
FixGrip the sleeve or hook under the leg on that side before committing.