Common Mistakes Catalogue

The mistakes below come up year after year. Make a flashcard for each — it's the highest-yield revision you can do.

Module A · Information Processing

Confusing validation (by software) with verification (by human).
Adding binary numbers without checking for overflow at the leftmost bit.
Mixing up Big-5 (Traditional Chinese) and GB (Simplified Chinese).
Forgetting that 1 KB = 1024 bytes in the IT context, not 1000.
Calling a pivot table the same as a chart — it's a table that aggregates.

Module B · Computer System

Saying RAM is non-volatile (it is volatile).
Saying ROM is volatile (it is non-volatile).
Treating firmware and OS as the same thing.
Listing Windows as application software (it is system software).
Forgetting that virtualisation is a mode of operation, not just VirtualBox.

Module C · Internet

Calling the Internet a LAN.
Mixing switch and router (switch = within a LAN, router = between networks).
Believing HTTPS guarantees safety (it secures transmission, not the site itself).
Saying viruses cause DDoS (more accurately: botnets, which may involve worms or trojans).
Forgetting that WPA2 is the password-based wireless protocol, WPA3 the newer.

Module D · Programming

Using = instead of == in pseudocode conditions.
Writing nested loops in the Compulsory Part (nested loop is Elective 2C, not Module D).
Producing a trace table that skips an iteration.
Forgetting to initialise a counter before a loop.
Treating int and string versions of the same number as interchangeable ("5" + 3 is an error in Python).

Calling all free software "open source" — freeware ≠ open source.
Believing buying a copy transfers copyright.
Treating AR and VR as the same.
Writing one-sided ethics answers (markers reward balanced view).
Forgetting to name a specific group affected by the digital divide.

Elective 2A · Databases

WHERE on aggregates instead of HAVING.
Cartesian product from missing ON clause.
JOIN direction wrong (LEFT vs RIGHT).
Designing M:N without an associative table.
Using reserved words as identifiers (e.g. ORDER, DATE).

Elective 2B · Web Application

Believing client-side validation is enough for security.
Storing passwords in plaintext.
Using GET for actions that change state.
Concatenating SQL strings (no prepared statement).
Confusing session with cookie.

Elective 2C · Algorithm

Modifying an array while iterating.
Off-by-one (range(n) vs range(1, n+1)).
Returning the wrong type (e.g. returning index when value asked).
Forgetting that binary search requires sorted input.
Not closing files (forgetting with open(...)).

SBA

Scope too large; nothing finished well.
Tests cover only happy path.
Evaluation is too positive — no honest reflection.
Missing backup of working files.
Late submission.

How to use this list

Print it as a 1-page cheat sheet.
After each mock exam, add 3 mistakes of your own.
Review weekly in S6.

➡️ Next: Programming pitfalls