1. Inconsistent style
Half APA, half Harvard, with elements in random order. Pick one system — the one your department asks for — and apply it everywhere, in the same format for every entry.
2. Missing in-text citations
A full list at the end is not enough. Every idea or fact that isn't yours needs an in-text citation that matches a reference-list entry — and vice versa.
3. Wrong or incomplete source details
A missing year, volume, pages or DOI/link. For online sources, add a DOI where one exists; it's more stable than a URL that can change.
4. Mixing APA & Harvard
They look alike but differ in the details: position of the year, "&" vs "and", italics. Mixing them is obvious to a marker. Keep one.
5. Wrong order and list formatting
The reference list goes alphabetically by surname, usually with a hanging indent. Don't number it unless the style requires it, and don't shuffle the order.
Worked examples in the referencing and citations guide.