The pattern
Group digits by place value, then do brackets and multiplication or division before addition or subtraction.

Large numbers add up correctly only when digits of the same place value sit under each other: ones under ones, tens under tens, hundreds under hundreds. Line the numbers up this way before adding, and carry any digit that makes a column reach ten into the column on its left. Ordering numbers compares the hundreds digit first, moving to tens only if the hundreds match.
Two numbers to add with carrying, several numbers to put in order, or a sum mixing addition, multiplication and brackets, all point here.
Children forget to carry a ten into the next column, or compare digits from the left without first checking the numbers have the same number of digits.
All the problem patterns → · the revision list
All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
A PDF, and a note when new solutions go up.