- incessantly duplicating already existing functionality: utility functions, UI components etc.
- skipping required parameters like passing current user/actor to DB-related functions
- completely ignoring large and small chunks of existing UI and UI-related functionality like layouts or existing styles
- using ad-hoc DB queries or even iterating over full datasets in memory instead of setting up proper DB queries
And so on and so forth.
YYMV of course depending on language and project