- Missing the base case (or not realizing there are multiple) - Not understanding/using variable scoping - Insufficiently-exhaustive case checking (especially

- Missing the base case (or not realizing there are multiple) - Not understanding/using variable scoping - Insufficiently-exhaustive case checking (especially if there are multiple variables) - Using built-in data-types that do not have the functionality or invariants that you need instead of making custom types --- (subcase) Using the wrong number type: Using floats for any non-integer, when sometimes you want Decimal or need to hand-roll a fixed-point number; Not thinking about the size of your integer type. --- (subcase) Encoding data as strings, parsing out and concating in as needed. For some reason, this is surprisingly common in 101-level beginners.