Problems

Recognise it when: the input is a grid to transform in place, or numbers too big (or too awkward) to handle directly.

• Rotate a square matrix 90° clockwise: transpose, then reverse each row. Layers work too.
• In-place markers: reuse the first row and column (or a sign bit) to remember what to change, so no extra matrix is needed.
• Digit arithmetic: walk from the least significant digit, carry to the next; a result of length len(a) + len(b) is enough for a product.
• Cycles in a sequence of numbers (e.g. repeatedly transforming a value): detect them with a set or with fast/slow pointers.

Common mistakes: changing cells you still need to read; forgetting the final carry; off-by-one on layer boundaries.