(1) sin vs cos: Line-plane angle uses sin, not cos. (2) Zero denominators: If a DR is zero, write the equation as a fixed coordinate (e.g., z = z_{1}), not as a division by zero. (3) Absolute value in distance: Always take absolute value for distances. The sign of ax_{1}+by_{1}+cz_{1}+d indicates which side of the plane the point lies on. (4) Parallel vs skew: Just because lines don't intersect doesn't mean they're skew — check if they're parallel first. (5) Family of planes: $P_{1}$ + lambda·$P_{2}$ = 0 doesn't include $P_{2}$ itself (lambda = infinity). If the answer turns out to be $P_{2}$, check separately. (6) Image vs foot: The image is at distance 2t from P, the foot is at distance t. Don't confuse the two.