Inequalities (linear and quadratic)

With a negative $x$- or $y$-coefficient, keeping the original sign, or replacing the inequality with an equals sign or a bare number. Dividing by a negative flips $<$ to $>$.

Quoting $-2<x<3$ when the answer is $x<-2$ or $x>3$ (or vice versa); not sketching, or writing factors that do not expand to the given quadratic.

Drawing the line for $x+y=7$ through the wrong intercepts, or not labelling the lines and not indicating the shaded region.

Using a dashed line for $\le$ or $\ge$, or a solid line for strict $<$ or $>$. Solid includes the boundary; dashed excludes it — the wrong style loses the accuracy mark.

For $-1\le x<3$ the integers are $-1,0,1,2$: include $-1$ (the $\le$ end) but exclude $3$ (the strict end). Slips add $3$ or drop $-1$.