(1) Forgetting that straight wire segments through the point of interest contribute zero field. (2) Using $N$ (total turns) instead of $n = N/L$ (turns per unit length) in the solenoid formula. (3) Wrong direction of field — always apply right-hand rule carefully. (4) Confusing force between parallel (attract) and antiparallel (repel) currents. (5) Using Ampere's law where there is no symmetry (it is always true but only useful for symmetric distributions). (6) Forgetting the factor of 2 in the infinite wire formula vs. semi-infinite.